Add the @keypath macro.

This macro can be used for compile-time verified keypaths, in place
of simply using string constants such as when setting up parsers or
KVO.

It is based on the version in EXTobjc found in Reactive Cocoa, but
uses the SetDirection macro dispatch helpers instead of the EXTobjc
metamacros.
Merge pull request #124 from walmartlabs-samsclub/feature/keypath-macro

Add the @keypath macro.