Adds usagestats
Records general usage info
Records info from 'reprozip pack'
Disables usage reports during tests
Makes Travis install usagestats from Github
Merge branch 'master' into 'usage-stats'

Conflicts:
	.travis/install.sh
	reprounzip/reprounzip/main.py
Sets droppoint and certificate for usage reports
Merge branch 'master' into 'usage-stats'

Conflicts:
	reprounzip/reprounzip/common.py
	reprozip/reprozip/common.py
Adds pack_id uuid in canonical config
Factors record_usage_package()
Merge branch 'master' into 'usage-stats'
Records run['argv'][0] for each run in pack
Adds REPROZIP_USAGE_URL env var
Records program name with version
Adds missing submit_...() call
Records pack details in load_config(file)
Don't submit report on UsageError
Adds 'usage_report' commands to enable/disable
Stores pre_setup and pre_run signals
Records more data