Add a new category to cpplint to detect missing white-space in comments

Currently we haven't turned on any rule in cpplint.py to detect a missing leading whitespace in comments. Newbies tend to make this mistake even while trying to follow the style guide.

There is already a category in cpplint.py (whitespace/comments) to detect this, but it has the unfortunate side-affect of also tagging any trailing comments with less than 2 whitespaces between the code and the trailing comment as errors. This side-affect generates a lot of false-positives since trailing comments in mesos codebase usually has only a single white-space. Hence proposing adding a new category called whitespace/mesos-comments that takes care of the more common case of tagging comments with leading whitespace as errors.