importing bittrex.bittrex vs bittrex

[ericsomdahl]

in PR #104 (as was pointed out by @skyl here) I let slip by what should be considered a style regression:

we have added code into the bittrex.init.py that automagically brings in the bittrex.py file. Importing bittrex.bittrex is redundant and I propose we standardize on the shorter form of the import.