Fix leaky "from ggplot import *"

Issue
-----
>>> from ggplot import *
brings in module level global variables

Problem
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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

Solution
--------
1. __all__ instead of __ALL__
2. Encode strings in __all__ as ascii

References
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1. Commit 0cedbacf6917c2ccd0cc81be2100cc747706984e
1. Github Issue #207 (https://github.com/yhat/ggplot/issues/207)
2. http://stackoverflow.com/questions/19913653/no-unicode-in-all-for-a-packages-init