Create parent class for import

Currently there are 5 groups of import class:
CRM_Import_*
CRM_Event_Import_*
CRM_Activity_Import_*
CRM_Contribute_Import_*
CRM_Member_Import_*

They are all very similar, but do not share or re-use any of each other's code – essentially they are copies of each other with a few variables changed. 5 copies of the same code is becoming increasingly difficult to manage, as fixes going into one don't go into the other 4, etc.

Proposed solution:
1) Rename/move CRM_Import to CRM_Contact_Import so it's naming is consistent with the other 4.
2) Make CRM_Import a parent class that all 5 can extend.
3) Move common functionality into the parent.