Added documentation on setting process variables.

* Moved main documentation to VariableScope#setVariable() and
VariableScope#setVariables();
* Updated descriptions for RuntimeService#messageEventReceived(),
RuntimeService#setVariables() and RuntimeService#setVariable to point to
main documentation;
Merge pull request #190 from tiesebarrell/master

Added documentation on setting process variables.
Merge remote-tracking branch 'upstream/master'