Support expressions in @media attributes
Allow float degrees / percentages in color functions
Use greyscale rather than grayscale.

The latter doesn't exist (anymore).
Apply bootstrap3 theme CSS formatting

The previous test case was generated by lessc (upstream). Meanwhile,
lesscpy also parses it but generates slightly different (though
functionally equivalent) output. Therefore adjust the desired output so
that this test finally passes.
Output rgba(x,y,z,0) exactly as lessc

Currently a hack.