initial commit
code reorganization
complex expressions
filter functions can return None to skip this token
Token classes can skip tokens by raising TokenError in init() method
Selector token accepts lists of selectors now
Color token accepts color names now
:nth-child() series of selectors supported
* (0,1) means tools.opt()
Expression class evaluates expressions with unit conversion
rendering system
unified filters interface, added repl(string) filter
better filters interface in Parser class

parser[index] -> filter through take(index),
parser / string -> filter through repl(string)
scale units conversion added (em/ex/%)

em<->ex conversion is a rough approximation based on heuristic analysis.
parent=None -> context={}
a little clearer basic tokens definition
don't wrap varval into Value class

Correct wrapping is determined based on variable type (global vs. local)
use new syntax sugar to make statement tokens definition a little clearer
correct recursive rendering of rules
Parser class supports slices