code optimization

[amateja]

Code is simplified with use of list and dict comprehensions and bit-wise operators. It should be also a bit faster. No library functional logic is changed but two minor breaking changes should be noted here:

• bitoptions.utils.number2powers returns a generator not a list, values are now sorted in ascending order
• bitoptions.utils.BitOptions._initialize_lookup method is removed.

[coveralls]

Coverage remained the same at 100.0% when pulling 80aeb04 on feature/optimization into 5caae0c on develop.