Add JSON support for output format.

JSON format is good for sharing data with other languages, which makes
it easy to integrate pgBadger's result into other monitoring tools
like Cacti or Graphite.
Add execute attribute to pgbadger in the source repository, some may find this more helpful when pgbadger is not installed and executed directly from this repository.
Merge branch 'master' of github.com:lansz/pgbadger into lansz-master
Merge branch 'lansz-master'
Add minimal documentation about JSON output format.
Fix missing database name in samples of top queries reports. Thanks to Thomas Reiss for the report.
Fix overcount of sessions when a client attempts to log in and is kicked off because of failed authentication or an incorrect database name. These show up as sessions which connected and never disconnected. Thanks to Josh Berkus for the report.
Fix remote ssh command error sh: 2: Syntax error: "|" unexpected. Thanks to Herve Werner for the report.