Added more documentation for mappers. #17
More documentation tweaks.
Added notion of accessors.

KeyMapping now uses HYDAccessor protocol to fetch extract values from an
object. The two obvious ones are:

- KeyAccessor that uses -[valueForKey:]
- KeyPathAccessor that uses -[valueForKeyPath:]

Both accessors can support "batch" fetch of multiple arrays.
The key mapper only reads the first value returned.
Added test coverage around multiple fields (and single fields).
Added accessor tests
Refactor. Most core mappers can accept HYDMapper objects as an alternative first arg.

#19.
Converted remaining few mappers to accept HYDMapper. Closes #19.
Whitespace trim
Added better descriptions for errors for easier debuggability. Closes #10.
Bumped hydrant version
HYDKeyValueMapper -> HYDObjectMapper.

 - Made HYDKeyAccessor emit fieldNames that are quoted when a dot exists
   in it.
 - Changed prefix of spec helper files to HYDS to make it more evident
in specs.
 - Removed unused code.
Fixed failing import
Updated cedar version
Fixed value transformer mappers to use mappers internally
Fixed HYDSPerson not checking for homepage property.
Fixed indentation
Fixed typed mapper returning nil

When the inner mapper returns a non-fatal error.
Fixed RFC3339 constant.
WIP: added reflective mapper.

Supports introspection into ObjC Classes to
determine the mapping required for HYDObjectMapper.

- Added some descriptions for debug ability.
Added basic descriptions to mappers.