preserve alias case while lowercasing taxonomy

* Changed MakePath so that it preserves original case. 
* Added a MakePathToLower that lowercases the generated path. 
* Modified kp in taxonomy.go to call the new MakePathToLower instead of the original MakePath.
* Added and modified tests for MakePath and MakePathToLower.

Additional calls to MakePath that were not changed:
* helpers/url.go line 36 
* target/htmlredirect.go line 43 

The affect on tags is unknown.

Affects: https://github.com/spf13/hugo/issues/329