Added the 'variables' function which returns all variables in a uri template.

Example usage:

>>> uritemplate.variables('http:www{.domain*}{/top,next}{?q:20}')
>>> set(['domain', 'next', 'q', 'top'])
Added tests for 'variables' function
Merge pull request #7 from deontologician/master

Added a new function "variables" which returns all variables from a uri template