Add dialyxir and fix all the typespecs

[doughsay]

I was trying to use dialyzer in a project of mine using this library and noticed errors that I couldn't fix without touching this library. Here are some fixes I think will help, but there's plenty of room for other improvements. At least I think this is a good start.

The main issues:

• Map.t doesn't exist, it should just be map
• [] I think matches only "empty list". [any] is a generic "list of zero or more anything"
• The exception stuff was a bit squirrelly, I opted to just treat it like a behavior implementation
• Because new/1 was being defined in Braintree.Construction, I removed the typespecs for all the overridden ones, because dialyzer can't handle more than one typespec per function

Hope this helps, let me know if you'd like anything changed! ;)

[sorentwo]

This is awesome! Thank you for taking the time, and having the diligence, to see all of these fixes through. The only bit I'd rather not have in here is the version bump, but as it isn't being released yet anyhow I don't see any harm.

[doughsay]

Sorry! My mistake, the version bump was not meant to be there. Thanks for merging though!