Fix RestClient::Request docs when used with yard

This moves a backwards compatibility comment.  It's currently replacing
the class documentation, which means that most of the docs for this
class are missing on rubydoc.info -
http://rubydoc.info/gems/rest-client/RestClient/Request
Merge pull request #319 from alext/fix_restclient_response_docs

Fix RestClient::Request docs when used with yard
Rename Platform.mac? to .mac_mri? for clarity.

This better reflects the fact that the method is expected to return
false on non-MRI engines even on OS X.
Merge pull request #323 from rest-client/accept-star

Change default headers to Accept: */*.
Add a more descriptive rest-client user agent.

Identify ourselves as rest-client and also include system architecture
and ruby / ruby engine version information.

For example:

    User-Agent: rest-client/1.7.2 (linux-gnu x86_64) ruby/1.9.3p550

    User-Agent: rest-client/1.7.2 (linux x86_64) jruby/1.7.9 (1.9.3p392)