Drush timeout option and return value

[Rade333]

Proposed changes:

• add timeout option to enable running time-consuming commands
• introduce new parameter $return_process, to allow caller to choose which return value runDrush() returns: the normal string output, or the whole Process object. This is needed since some drush commands put their output to STDERR instead of STDOUT. See examples of usage in updated README. The new variable is optional, and default to FALSE, to maintain backwards compability.