Add Carthage support

[SSheldon]

Not everyone uses CocoaPods! Should be relatively easy to add support for Carthage.

[SSheldon]

This was originally mentioned by @simonnarang in #38.

[SSheldon]

Not as easy as I hoped. Ran into a couple issues:

• TDOAuth doesn't currently build on Carthage, it hits a linker error for "embedded dylibs/frameworks only run on iOS 8 or later". The deployment target for TDOAuth is set to iOS6, and apparently that now causes build problems.

  This isn't such a big problem; normally I'd just send a PR fixing this to TDOAuth. Except support for TDOAuth appears to have been dropped recently :/ It's been moved from tweetdeck into an org called "tweetdeck-archive", so I'm not confident they'd even accept a PR. Instead we might have to go through the process of forking it.

• Supporting Carthage appears to also require that you use Carthage for development :( We're more comfortable with CocoaPods and would like to keep using it in development, but the project won't build on Carthage unless you link frameworks from Carthage/Build/iOS. I was hoping to find a workaround for this, but all I can think is having separate projects or duplicate schemes or something like that. I'm not psyched about any of the options.