Stubs of parsers for syntax data types

Define stubs of parsers for all data types declared in the Language.Css.Syntax
module.
Half-hearted progress with @import, "" and url()

The stringp, urlp and atimportp parsers can now all parse *something*, though
they are mostly wrong w.r.t. the fine details. More tomorrow.
Tweak some comments, skip some spaces, stub some selectors

Tweak some of the comments (make examples code, etc.) and sprinkle skipSpace in
some of the existing parsers. Add stubs for some parts of selector parsing.
Start test suite for parsers

Add a test suite to the cabal file and begin implementing QuickCheck properties
for the Parse module. This includes:

- Arbitrary instances for some of the Syntax data types.
- A property to check the pretty-print/parse round-trip.
Ignore
Add unittest runner
Specify other-modules in test-suite

This ensures that HPC profiling works correctly.
Convert into a separate package
Add properties for color and uri
Fill in parsers for the basic value types
Merge branch 'feature-parse' of github.com:thsutton/language-css into feature-parse
Add Unittests module to cabal file

This prevents HPC from trying to profile it.
Always pick doubles with ".0" to avoid fixing test failures
Add property for @import rules
Parsing improvements

- Make keywords case-insensitive
- Skip white space in some places
- Parser for expressions
- Better parser for identifiers