Fixing INCRBY inconsistent naming

According to redis documentation there are two commands:
INCR - (incrementing by 1)
INCRBY - (incrementing by N)

Redis-py client implements it in the least obvious way: there is `incr()` that requests INCRBY redis command, and there is no `incrby()` method.

This is very confusing, as Redis-py README asks for referencing original Redis commands documentation for actions names.

This convention is also not consistent in the client itself. For example HINCRBY is called `hincrby()` method, rather than `hincr()` (as could be suspected?)

This fix just adds `incrby()` method that is an aliast for `incr()` method (as both already are using INCRBY) to maintain maximum backward compatibility but also be less confusing for developers referencing original redis command documentation.