Much improved variable expansion

With this change variables can be used only in part of a rule value, for
example "padding: $top $left;" will now get properly expanded.

This still doesn't handle the case of `rgba(255, 255, 255, $foo)`, as
the value is being split on spaces, but let's address that later when we
get a failing spec with the case.
Remove trailing newline inconsistencies.

This is a temporary fix so that we don't get test failures for trailing
new line where it's not supposed to be. We can figure out a more correct
way to do this later, as currently I don't really understand the rules
for trailing newlintes at the end of a file.
Add prefix selector to flatten

This way we can keep building up the nested selector prefix chain to
arbitrary depth.
Add comment parser
Add split package dependency
Add minification to Compiler
Minify integration tests

Given that the default SCSS indentation in the sample files is different
from what the current printer produces, it's easier to just minify both
files and compare the content.
Support comments
Add parser for grouped selectors

For example "h1, h2, h3"
Fix types import
Update README with current progress
Add comment parser specs
Remove dead code
Merge pull request #5 from darthdeus/variable-expansion

Variable expansion
Fix Selector monoid spacing

Previously the values around "," would get joined without a trailing
space, resulting in "p,q" instead of "p, q"
Allow additional selector combinators, such as +~*
Whitespace cleanup
Add regexp compact replacement