Fix leaky "from ggplot import *"

Issue
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`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

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
2. http://stackoverflow.com/questions/19913653/no-unicode-in-all-for-a-packages-init