This fixes issue #152 by enhancing the current set of metrics captured
by mysql check. The current set ofi MySQL metrics provided by Datadog
is very limited (based on my experience as a Senior MySQL Consultant
at Percona).

The following changes have been made to the mysql check:
a. Its heavily inspired by MySQL cacti template provided by Percona
which contains an extensive set of checks that are important to measure
the performance and health of a MySQL server. For reference please look
at this URL:
http://www.percona.com/doc/percona-monitoring-plugins/1.1/cacti/mysql-templates.html#sample-graphs

b. Many of the important metrics exposed by Percona Server / MariaDB Server are
not available in MySQL Server as status variables, these have been added
by parsing the output of "SHOW ENGINE INNODB STATUS"