Add missing import statements.

Issue
-----
Code especially in the `ggplot/tests/` directory
was using unavailable modules. Many tests that
passed depended of leaked imports which are no
longer available.

Problem
-------
Various missing import statements

Solution
--------
Add the import statements
Python3 does not like to import byte literals

Issue
-----
For the import * statment

unicode literals as variable names => bad for python2, ok for python3
byte objects as variable names     => ok for python2,  bad for python3
str objects as variable names      => ok for python2,  ok for python3

Solution
--------
Convert all objects in __all__ to type str