Add argb() function

Used by bootstrap3. Quote from the lesscss.org documentation:

Creates a hex representation of a color in #AARRGGBB format (NOT
Android development.
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.
Skip empty @media blocks
Accept resolution types as css_number

dpi, dpcm and dppx, check http://www.w3.org/TR/css3-values/#resolution
Proper media queries

The lexer now identifies media queries in @media ... {} and
@import ... ; statements. For that, two new states "mediaquery" and
import" where added. The parser uses those to implement the CSS3 media
query BNF (www.w3.org/TR/css3-mediaqueries). Overall test coverage
increased.
Implement nested media queries

With proper test coverage of all the various cases of
http://www.lesscss.org/#-nested-media-queries
fixup!