Allow special characters in target names to be escaped

[mleinart]

I think this is primarily a change to the pyparsing grammar for targets (https://github.com/graphite-project/graphite-web/blob/master/webapp/graphite/render/grammar.py).

Though the browse tree code should also know how to deal with a target name like this properly once a fix is in place in the rendering code.
#120 is an option as well, but I feel like url encoding doesnt really belong that far down from the view code. Instead, I think we can solve it by starting to treat the targets more as we would other grammars and just 'backslash it out'

e.g. a metric named

systems.web01.load(5min)

would be added as:

systems.web01.load\(5min\)

[mingbowan]

what about file system paths? like "/usr/local"? You cannot escape "/" in file path

[felixbarny]

👍