Add user agent

GitHub recently added a User-Agent requirement to their API:

http://developer.github.com/changes/2013-04-24-user-agent-required/
Merge pull request #14 from pufuwozu/user-agent

Add user agent
Removing exceptions, adding Either
Moving OAuth2.hs to Internal.hs (reexporting common things through OAuth2.hs later)
* purified a few functions
* lots of moving around
* replaced exceptions with Either BS.ByteString a (type synonymed to OAuth2Result in Internal.hs)
Updated Cabal file with module structure
Updated tested GHC versions
merge part change of tazjin and bump to 0.3.0
bump up to 0.3.2
Purify httpclient functions and update examples
remove comments
simplify auth request API
update comments
Fixed type error
example google offline fix
Merge pull request #15 from gertcuykens/master

example google offline fix
Fixed issue #17
updates for new aeson, http-conduit