Added documentation on setting process variables.

Updated the documentation and linked from places indicated by Joram in https://github.com/Activiti/Activiti/pull/187

I have applied formatting to both updated classes, which, to my surprise, resulted in a huge number of changes, especially in RuntimeService. However, I believe I used the formatting rules as provided in the eclipse resources folder for the project. Is this OK?

* Moved main documentation to VariableScope#setVariable() and
VariableScope#setVariables();
* Updated descriptions for RuntimeService#messageEventReceived(),
RuntimeService#setVariables() and RuntimeService#setVariable to point to
main documentation;