Add to list of valid chars in path_segment

[talbright]

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

[talbright]

@steveakers I'm going to create PR's from now on instead of merging directly into master. This allows Travis to run on the PR, provides more visibility and tracking, and gives us a chance to discuss the PR before merging if necessary.

Once the build passes I'll handle the merging myself, so you don't need to do anything.

[coveralls]

Coverage remained the same when pulling dc61680 on talbright:fix-parser-wilcards into 176cbcd on livingsocial:master.