Make some variables global.

This fixes a few issues that are created when random RoundTrippers get
created during the test cycle. By making these global we ensure that DVR
is a singleton which is what is expected.
Add Gzip support to output files.

This adds gzip support to the output file generated. This is necessary
since the files can get huge otherwise. Specifically this reduces a 25MB
file into a 900k file for a related test I wrote.
Merge pull request #1 from orchestrate-io/gzip

Add Gzip support to output files.