Support backslash escape of grammar characters

... except for dots ( . ) and braces ( {} ) which will require work in evaluator.py at least

This allows for metrics with special characters such as parenthesis and commas e.g.:

    averageSeries(servers.www01.loadAvg\({5,10,15}min\))
    servers.www01.cpu0\,1\,2\,3.user