Report stats to server.

Specify stats_upload_url to pants.ini to receive a
POST form with 'cumulative_timings', 'self_timings',
'artifact_cache_timings', and 'run_info' params, each of
which have a JSON body.
Stats upload review fixes.
More review fixes.
Merge pull request #19 from foursquare/reporting

Report stats to server.

Specify stats_upload_url to pants.ini to receive a
POST form with 'cumulative_timings', 'self_timings',
'artifact_cache_timings', and 'run_info' params, each of
which have a JSON body.