Add to list of valid chars in path_segment

Allow path segments like:

host.cpu-[0-7].cpu-{user,system}.value

Please note that the parser is not strict. For example it would allow:

host.cpu-][.cpu-}{.value

Fixes livingsocial/rearview#42

See:
http://graphite.readthedocs.org/en/0.9.10/functions.html#graphite.render.functions.averageSeriesWithWildcards