added `from __future__ import absolute_import`. Clearer imports.

In Python 3, you cannot have an ambiguous `import X` if X is
both a module and a subpackage/submodule. By importing this here,
we get rid of ridiculous relative imports like `from ... import X`.
Merge pull request #80 from plotly/future-imports

added `from __future__ import absolute_import`. Clearer imports.