Updated gem dependencies, achieved Ruby 2.0 compatibility

My final concern was to see if and create Ruby 2.0 compatiblity if not yet present. On this way I had to apply some smaller and some more radical changes to the project (e.g. removing rcov support since rcov does not work with Ruby 1.9 and above) so I'd understand if you have some doubts on merging this right away - I will be happy to make further adjustments!

What I've done:
* since `bundle install` (with development gems) failed under 2.0 I went back checking the dependencies and updating them/removing the contraints to old versions
* afterwards I installed new versions and executed the test suite, many regressions as I feared, most of them caused by new versions of HTTParty and deps
* first class of errors came from changed behavior of MultiJson that now tries to parse error documents to they are not always application/json (did some checks against the current CloudApp API, they changed things there too)
* second class of errors came from broken date/time parsing, that was previously done by Crack JSON parser automatically but that is now replaced by MultiJson (although one can force HTTParty to use Crack again...) so I invented some explicit date time parsing on json attributes that end in "xy_at"
* I finally added a travis-ci config to automatically test against Ruby 1.8.7, 1.9.3 and 2.0.0 that is currently only available for my fork: https://travis-ci.org/cmur2/cloudapp_api

Hope you like it!