Add g1gc collector

G1 GC collector:
    script that scan G1 GC log and populate OpenTSDBmetrics into stdout

Dependencies:
    linux command: since
    python package: docopt

How it Works:
  1. Find the latest gc log under GC log directory (specified in "--log")
  2. Retrieve latest N lines of GC log (specified in "--limit")
  3. Write these N lines to be parsed into a tmp file (specified in "--tmp")
  4. Update since file (specified in "--since")
  5. Parse the log and put metrics into stdout, metric name will be prefied if "--prefix" is specified
  6. Remove this tmp file (specified in "--tmp")
  7. All errors happened during parsing will be logged in error file (specified in "--error")