Rename some relative date filters to accurately describe them

Possibly this will get split into two issues, but ( I think) it is much easier to explain what I want to do and why in one description.

At the moment there is a set of relative date filters that are described as "Prior to Previous 'time interval'". I think they are misnamed and need to be renamed. I also want to create a set of time filters that DO match with the existing names.

Taking TIme interval = month as an example.

I will take the day I am creating this issue as the current date ie 26 March 2015.

When I see a relative date filter with the name "Prior to previous month" I expect it to include all dates up to and including 31 Jan 2015. Reasoning: February is the previous month and so "prior to previous month" should include all dates prior 1 February 2015.

What this filter will actually include is only the days from 1 January 2015 to 31 January. Ie the "MONTH prior to the previous month".

Possibly whoever named this relative interval was trying both to avoid using 'month' twice and to keep the description short, but I think the wrong 'month' was left out. I think "Month prior to previous month" can be abbreviated to "Month prior to previous" without changing the meaning, but "Prior to previous month" means something else entirely as I have already outlined.

All the other relative date filters in that set work in the same way.ie they filter on "Day prior to previous day". "Week prior to previous week" etc

I am proposing two changes to core.

A. Renaming the existing filters to better describe the actual time intervals they filter on.
The alternatives I can think of are:
1. Month prior to previous month
2. Month prior to previous
3. Second last month

I think 1 is the most accurate but is wordy. I would be inclined to chose 3. over 2. as non-native speakers may misinterpret 2. (which is perhaps another explanation for the current name)

B. Create a new set of time filters that actually do what the existing names describe. ie create filters that include all dates up to the beginning of the previous day, week, month, quarter or year.

I understand that using the existing names for these new filters could create confusion, so I am proposing new names. The alternatives I can think of are:
1. All dates prior to previous month
2. All prior to previous month.
3. To start of previous month.

I think 1. is the the most accurate but is wordy. I am hoping that 2. will be interpreted correctly. 3. "sounds" nicest, but will people be think 1 February should be included?

I know the list of relative dates is growing all the time, but I have a need for the "All prior to previous month" filter which is how I discovered that "Prior to previous month" is not an accurate description of the existing filter. I would be happy to add just that one if that is thought to be better under the circumstances.

Also, yet again I am hoping to sneak this in to 4.6. so I will probably submit an initial PR in the next couple of hours.