2.1.0: A Little Bit of This and A Little Bitcode of That

Argo 2.1.0 coming at you!

[NEW]: decodeWithRootKey convenience function for easy root key decoding
[NEW]: Custom error case Decoded.Custom(String)
[NEW]: Failure Coalescing Operator for Decoded, thanks @gfontenot
[NEW]: catDecoded for Decoded, thanks @gfontenot
[NEW]: DecodeError convenience accessor in Decoded, thanks @jshier
[NEW]: Default Decodable implementations for String and Int RawRepresentable enums, thanks @swizzlr

[IMPROVEMENT]: Documentation!!!!, thanks @jnutting & @gfontenot
[IMPROVEMENT]: Extend CollectionType instead of Array, thanks @gfontenot & @gilesvangruisen
[IMPROVEMENT]: Extend DictionaryLiteralConvertable instead of Dictionary, thanks @gilesvangruisen

[BUG FIX]: Fix bitcode error when submitting app, thanks @dmcrodrigues

Big win!