Python 3 compatibility and pep8 error fixes

While trying to install this package to use with our Py3 project, saw that it was breaking because it wasn't using the relative import. So added that. Also added a simple test case. And also testing with Travis that it works with Python 2.6 to 3.4 - [![](https://travis-ci.org/iambibhas/instamojo-py.svg?branch=master)](https://travis-ci.org/iambibhas/instamojo-py)
Also, fixed a lot of pep8 errors, except the 80 char limit and unused import ones.