Move temporary build directory creation into common
Refactor plugin release script

Now it:
	* Has a help output when invoked without arguments
	* Makes use of the settings in freenetrc
	* Uses intermediate variables for maintainability and readability.
	* Inserts directly instead of assuming it's on the same host as the
	  node
	* Uses different exit codes for different situations
	* Uses in-place sed on the webserver
	* Has whitespace to help with readability
	* Documents reasoning behind ignoring underscore and beyond in
	  build product names.