Added HOM version of RAC lifting

- added rac_lift and associated private RACLiftProxy.
- Refactored existing -rac_liftSelector:withObjects: and rac_lift in terms of
  common functionality. Tests pass.
- Added -[NSInvocation(RACTypeParsing) rac_argumentAtIndex:] which behaves like
  -rac_returnValue.
Added specs for -rac_lift HOM
RACLiftProxy: set NULL as return value for non-object return types
Fixed copy-pasta mistake

Was -getReturnValue:, should have been -getArgument:atIndex:
Remove case for void argument type in -rac_argumentAtIndex:

Of course that case could never occur.
Corrected typo in -rac_argumentAtIndex: documentation
Added commentary to RACLiftProxy
RACLiftProxy: assert that it can only receive void or object-returning messages
Improved documentation for -rac_lift in light of changes
Factored out common behavior in RACLifting specs

Unfortunately, this doesn't bring down the line-count by much at all; but it
does at least avoid duplication of test cases.
Added spec that -rac_lift HOM should fail for messages returning primitive type
Correct typo in RAFLiftProxy assertion
Style cleanup in NSObjectRACLiftingSpec

- consistent use dot syntax
- consistent use new instead of alloc.init
Remove test for RAFLiftProxy HOM failure on primitive return types
Renamed method in <RACLiftingTestRig> for consistency with RACTestObject
Clarified documentation of <RACLiftingTestRig>
Removed no-longer-necessary method on RACTestObject

Since we're not testing failure of HOM for messages returning primitive types,
no need for this kit anymore.
Merge pull request #424 from jonsterling/rac-lifting-HOM

Added HOM version of RAC message lifting