Don't break HttpCache when body has unicode chars

The default serializer MultiJson is not happy with serializing UTF8
characters; switching to Marshal solves the issue
Specify all the gem dependencies

Some dependencies were missing from the .gemspec since they were
already installed on my machine -- now they are all there. Some
ActiveSupport methods have been replaced with default Ruby ones
Add specs :) Both local (gigs) and remote (github)

Move the content of github.com/rspec-api/gigs into the spec folder.
Now running `rspec` tests the `rspec-api` features both against a
local spec/dummy (a Rails app listing concerts) and against the
remote GitHub API.

Unit/function tests will be coming later, but for now this shows
how rspec-api works!

In order to run the github specs, a local .github file with the
Github token is required
Add badges to README