Expose middle_name property for individual contact object

Provide access to existing crm_individual.middle_name property in UI, and other interfaces:

1. Insert/update/getContact API's should include support for middle_name property (this may already be done...).

2. Add middle_name to 'importable' fields array, so it can be mapped and imported.

3. Middle name should also be exposed as mappable for CiviCRM Profile (i think this happens automatically when #2 is done ??).

4. Modify Add/Edit Individual form/template to include middle name. (validation=not required)

NOTE: I think we'll need to re-org that part of the form for this change. You can work w/ Manish to do so. Let's try eliminating the "First/Last" label and using the entire width of the fieldset for the 'name' line. Then we can use text w/ class=description to underwrite each field with it's 'label':

[Mr. ] [David ] [Gregory ] [Greenberg ] [Jr. ]
Prefix First Name Middle Name Last Name Suffix