Add support for the Logos language.

Logos is a language implemented on top of Objective-C and sees widespread use in the iPhone jailbreaking community.
Its main purpose is to make the creation of Objective-C-method-implementation-replacing "tweaks" easier.

Pygments support awaits the merging of a pull request (https://bitbucket.org/birkenfeld/pygments-main/pull-request/130/add-a-lexer-for-the-logos-language). The Travis build will likely fail until the aforementioned pull request is merged.

The included example is the example from the Pygments repository. A better example is at https://github.com/rpetrich/DietBar/blob/master/Tweak.x.