Added bundle history and bundle contents capabilities

Hey,

Starting to work on the bundle API, put in reading the bundle_history and listing the bundle contents. You can test it out with:

u=Bitly::V3::User.new(OAuth2::AccessToken.new(nil,"YOUR_GENERIC_ACCESS_TOKEN"))
bh=u.bundle_history
bh[0].links

Tests:
I wrote a very very basic test but it fails because of a wierd error I can't figure out- it doesn't parse the JSON I saved in the fixture the same way it parses the response from the server,  and as a result it can't read the correct status_code (200) it expects. I saw you left a note on the fact you didn't include V3 tests. I imagine something works slightly differently? Anyway I did test it on my own user and if you uncomment the line in the bundle_test you can see what fails- if you have a hint I'll fix the test and all future tests that way.