Use classes to define standard field types and implement an "int" type

Summary:
Currently, ManiphestAuxiliaryFieldDefaultSpecification uses about a dozen giant switch statements to implement stadard field types (int, string, date, bool, select, user, remarkup, etc). This is:

• pretty gross;
• not extensible; and
• doesn't really let us share that much code.

I got about halfway through porting a similar implementation into StandardField but I wasn't thrilled with it. Subclass StandardField instead to implement custom field types.

Test Plan: Added an "int" custom field, verified it had integer semantics and indexed into the integer index.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D7005