Improve support for percent escaped url parameters (txmt://)

Previously we would let NSURL parse the parameter and ask for the path component. This however does not support tilde as shorthand for the user’s home directory, and certainly not with TextMate 1.x’s questionable syntax (using ‘file://~/«file»’ instead of ‘file://localhost/~/«file»’ or possibly ‘file:///~/«file»’ although this one seems a little wrong).

As for percent escapes, the issue here is that if the URL parameter escapes slashes or colons, not decoding the URL parameter before creating an NSURL instance will fail (as the URL is malformed). If however we do decode percent escapes and the URL parameter contains spaces, then NSURL will also fail, as a URL with spaces is malformed.

So I ended up bypassing NSURL and check for a few variants of a ‘file://’ prefix (which is also what TextMate 1.x did).

This closes issue #602.