10X performance improvement by re-using iframe

[brucejo75]

See #6 & #19.

Re-using the iFrame boosted the performance significantly.

The implementation:

1. caches the iframe.
2. makes sure the window context of the iframe is left clean after use.

Passed all tests, run as described in #11.

I have an application that calls vm.runInNewContext 100's of time per display loop. Recreating and inserting the iframe is killing performance. Much better with a cached iframe