Revisions to contributions docs to add CLA forms and correct Issue Tracker links

These revisions address issues #114 and #115.
https://github.com/jclouds/jclouds.github.com/issues/114
https://github.com/jclouds/jclouds.github.com/issues/115

Added sections on the Contributor License Agreement to Contributing to jclouds and Contributing to Documentation.

Corrected links to the issue tracker.  Added jclouds Issue Tracker link to Contributing to jclouds.

Corrected a few typos.