changed parse of CSS selector attribute names to allow mixed case

see issue 334, https://github.com/dotless/dotless/issues/334 ,

if following CSS specified, 

    img[imgType="sort"]
    { ...

then compiler was give error...

   Expected ']' but found 'y' on line XXXX in file '....':

Changing to 

    img[imgtype="sort"]

would get around issue.
Merge pull request #335 from ronl/patch-1

changed parse of CSS selector attribute names to allow mixed case