Fixed HYDURLFormatter to not crash on non-strings.

Closes #9.
Added HYDUUIDFormatter.

Allows conversions between UUIDs and strings using:

- HYDUUIDFormatter
- HYDMapUUIDToString
- HYDMapStringToUUID

Also refactored HYDSetError to HYDSetObjectPointer and can
be used for more than just HYDErrors.

Closes #5.
Refactored code.

- Used HYDSetObjectPointer in more locations.
- Used HYDLocalizedStringFormat in more locations.
Forgot about mutability
Fixed bugs with high-order mappers to capture the return values of child mappers that emit non-fatal errors
Removed spec header from Hydrant.h
Fixed missing import
Parsing an object with missing fields will return an error by default
A non-fatal error + nil value results in dropping the item from the collection in a collection mapper
Passing nil to HYDCollectionMapper will result in an error
Backfilled some HYDErrors.
Added custom error description that doesn't print out memory addresses
Added missing typecast
Bumped hydrant podspec version
Added missing null object pointer check
NonFatal mapper does not use the default value when a non fatal error occurs in its child mapper
Added ability to reverse FirstMapper, it simply calls reverse on all its mappers but preserves its ordering.
Added post-processing mapper as escape hatch. Stricter parsing of collections.

Added more constructors for collections.
Updated comment docs. Added NSDictionary source alias for object mappers (KeyValue + Path).
Tweaked error output when printing description.