Add "length of employment" field

Inject some JS/jQuery on the "View Contact" screen to show Length of Employment field using
http://momentjs.com/docs/#/durations/humanize/ for a helper which will generate those strings

====Original=============
Currently, there is a field for "Initial Join Date" . We also want to display the length of employment in the same spot. It should display as either the #months or as a natural-language description ("3 weeks", "2 months", etc – see http://momentjs.com/docs/#/durations/humanize/ for a helper which generate those strings)

== Option A ==

• Add a custom data field, "Length of Employment", to the same custom-data group
• The field should not be editable in the UI
• The field should update automatically (using a trigger or hook – whenever the join-date changes)
• The field should update periodically (using a cron task)

== Option B ==

• Inject some JS/jQuery on the "View Contact" screen

== Tradeoff ==

Option A is more brittle (eg fails due to misconfigured cron task), but it creates a searchable field. Option B doesn't support searching but should be less prone to misconfiguration.