Allow case-sensitive check for token

ping @Ocramius @mac_nibblet

I realized that by default, SQL do case-insenstivie checks for token. This can be an issue as tokens are fetched by the token itself.

I couldn't find anything in Doctrine 2 mapping to force having utf8_bin (which makes case-senstivie checks instead of default utf8_general), so I used a columnDefinition.