Fixed spelling: "exponant" -> "exponent".

[boxed]

This is unfortunately a potentially a breaking change for users. Fixes #84

[Psycojoker]

Damn -_-'

Me and my awful English.

This is quite problematic because I've stated that I won't do any breaking release before 2.0 for stability reason...

I guess this'll just wait :confused:

Thanks for your contribution nevertheless!

[boxed]

Yea, this change would break mutmut too, so it must be in a major. Pretty unfortunate!