Added greater than and lower than assertion methods to the css() assertion method

Hi Sebastian,

I badly needed to check if a css value is lower than a certain value, so I added that feature into dalek (I also added greater than, just to have both ways ready). The docblocks are updated. By reading them you should be able to understand my idea and set up working assertions.

I'd really appreciate if you would review my changes and merge my pull request. If there's anything I need to improve before you can merge the pull request, please let me know.

Kind regards
Marc