clearer error if you try to call librato.increment() without having called librato.start()

[kevinburkeshyp]

The message you currently get is TypeError: Cannot read property 'prefix' of undefined which doesn't make it amazingly clear what failed. The undefined is the variable config.

[timanovsky]

My 2c: let actually call librato.increment but make it no-op if configure/start has not been called. Otherwise it is hard to run code in development without librato, and in production with.