Basic URL canonicalization. Fixes #84

[brson]

...#84.

No tests because there's no preexisting network tests.

[brson]

This may not actually fix #84. There is another rev-parse error that occurs when packages don't agree on the capitalization of source URLs, so e.g. when some people use PistonDevelopers and others use `pistondevelopers'.

[brson]

Think I'll throw in a few unit tests at least.

[brson]

I've also stripped trailing .git extensions. This is ready.

[alexcrichton]

Just in case we change the hashing algorithm in the future, could this test just test equality of all the urls when hashed? (don't actually look at the specific hash).

Could you also add a test for case insensitivity? The one above just calculates one hash but it's not clear from the test that the hash was generated from a lowercase url.

[brson]

I will, but this is following the existing pattern.

[alexcrichton]

r=me with a minor comment