TransformerKit 0.5.0

It's a copy of the [latest spec in the repo](https://github.com/mattt/TransformerKit/blob/6c6634054f7848d9c095415fd4e08f21181748d6/TransformerKit.podspec), but also I've removed `s.source_files = 'TransformerKit'`. Because of that line all source files were added to the project when you specify `pod 'TransformerKit'` and `pod spec lint` was failing because `Security` framework is needed for `TransformerKit/Data`, but wasn't added as dependency. Looks like now people need to specify what parts (subspecs) of the library do they need. For example in my case `pod 'TransformerKit/String'` works well.

/cc @mattt

P.S. @mattt please let me know if you want me to send a pull request to the TransformerKit's repo with the same spec update.