Ensure purge on /etc/diamond/collectors

It is undesirable to have non-Puppet managed collectors in /etc/diamond/collectors, so since we provide a way to manage those, we should purge any which aren't
Update install.pp

Add dependency on /etc/diamond and create that directory
Update install.pp

Add ownership to /etc/diamond and /etc/diamond/collectors
Parameterise purging diamond collectors, defaulting to false
Ensure consistent use of purge_collectors parameter
Add tests for purge_collectors parameter and lack thereof
Merge pull request #12 from vulpine/master

Ensure purge on /etc/diamond/collectors