Fix leaky "from ggplot import *"

Issue
-----
>>> from ggplot import *
brings in module level global variables

Problem
-------
1. __ALL__ is ignored by the * import
2.
    >>> from __future__ import unicode_literals
    makes all strings default to unicode. Therefore
    variable names in __ALL__ are unicode. But
    python 2 variable names are not unicode
3. Any consequences of solving the above?

Solution
--------
1. __all__ instead of __ALL__
2. only object of type str in __all__
3. Fix the sloppiness (missing import statements) that crept in
   but whose detection got obscured because of the leaks.

References
----------
1. Commit 0cedbacf6917c2ccd0cc81be2100cc747706984e
2. Github Issue #207 (https://github.com/yhat/ggplot/issues/207)
3. http://stackoverflow.com/questions/19913653/no-unicode-in-all-for-a-packages-init
Merge pull request #230 from has2k1/fix-import-leaks

Fix leaky "from ggplot import *"