Make usernames case-sensitive

[bartfeenstra]

Usernames are processed in a case-insensitive way for a good reason (maintainers often make capitalization errors), but this also means that people with the same names but different capitalization share commits. http://ericduran.github.io/drupalcores/#xano links to https://drupal.org/user/2519336/track, while all commit mentiones belong to https://drupal.org/project/user/Xano, for instance.

This probably won't be a problem for most account names (including Xano, which is mine), but it's an interesting question nonetheless.