Add RRD `xport' support.
Add missing `Exporter' step argument.
Make mutex global.

 * Move `graphMutex' to a main `mutex'.
 * Fix collisions between `graphv' and `xport' (not being thread safe).
Merge pull request #7 from vbatoufflet/xport

Add RRD `xport' support.
Update documentation link in README
Merge pull request #8 from garyburd/patch-1

Update documentation link in README