Made private httpclient members non-static and implemented dispose
Prevent request headers from growing indefinitely

Changed the way request headers are added. Before, request headers were
continuously appended by repeated calls to the force client. Fixes #18.
Merge branch 'master' of github.com:bregger/Force.com-Toolkit-for-NET into bregger-master
Updated formatting
Merge branch 'bregger-master'