add feature to enable/disable legacy namespacing

[StephanX]

@mheffner,

So after a couple hours of tracking down "where are my lost statistics," I was pointed to this 'feature.'

https://github.com/etsy/statsd/blob/master/docs/namespacing.md

Essentially, namespacing conventions differ between pre 0.50 and 0.50+. This PR enables a toggle to choose which convention is preferable.

-S

[StephanX]

Ah, right on. I've pushed an update that will only set the key if graphite is enabled.

[StephanX]

Didn't in testing (which is why I didn't notice it) but have removed. Good catch!

[mheffner]

Thanks!