[MRG] Fix relative imports & improve package discovery

[mohamed-ali]

Fixes #20.

As described in pep-0404:

In Python 3, implicit relative imports within packages are no longer available - only absolute imports and explicit relative imports are supported.

The solution to the recent import problems, that is compatible with both python2 and python3 is to use explicit relative import.

A more thorough discussion about this subject can be found here.

This PR:

• Fixes the above import problem.
• Improves package discovery.

[yahoocla]

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[mohamed-ali]

CLA signed.

[coveralls]

Coverage remained the same at 88.443% when pulling 1f5834d on mohamed-ali:fix-relative-imports into 0f17655 on yahoo:master.

[mohamed-ali]

@pumpikano could you have a look at this PR, it fixes the import problem in python3 and is compatible with both python2 & 3.