Account for trailing / in URLs when determining on-disk location. Fixes #84.

No tests because there's no preexisting network tests.
Add a hack to make github urls case-insensitive. Fixes #84
Add tests for URL canonicalization and canonicalize .git extensions
Future proof URL canonicalization tests
Merge pull request #88 from brson/fix

Basic URL canonicalization. Fixes #84
Revert "Basic URL canonicalization. Fixes #84"
Merge pull request #89 from rust-lang/revert-88-fix

Revert "Basic URL canonicalization. Fixes #84"