Add Environment.Clear

[gavv]

Add new method Environment.Clear which removes all values.

New method should have documentation comment with code example and be covered by tests.

[e-berman]

I'll give this one a go @gavv.

Sorry for the stupid question, but you want to keep the keys and only clear the values, correct?

[gavv]

Cool, thank you. Nope, sorry for misunderstanding, I meant just to remove everything, keys and values. To allow the user to reset state.