Fix for Windows where absolute path starts with two leading backslashes

The fix to issue #356 solved the error on Windows when the absolute path starts with a drive letter, however sometimes Windows also uses two leading backslashes for an absolute path. I noticed this is the case when running node.js on the Azure platform. The fix is to simply check for all 3 patterns in the absolute function in utils.js.

https://github.com/LearnBoost/stylus/issues/356