ERD plotting support

[eywalker]

The logic for creating and manipulating graphs that represent the ERD is pretty much there and can be found in the erd branch. I will keep this branch as the feature implementation branch. There are a few issues that I'm running into as I try implementing plotting support:

1. pygraphviz library that was once used in our project for plotting ERD does not support Python 3 natively yet. Explicitly using their beta branch with pip install pygraphviz==1.3rc2 allows us to use it in Python 3, but I'm not sure if we would like to introduce a dependency on beta-version of the package, especially as this is a very specific dependency.
2. pydot is an alternative package that may be used, but again there is no official Python 3 support. There appears to be a port pydot3k

The above two were the preferred method to work with graphviz graph plotting engine, that yields very nicely arranged graphs. Alternatively, I could try working with graphviz directly, but to my knowledge, the library can only render files like .pdf files at the end and it takes some effort to display this into matplotlib.

[dimitri-yatsenko]

Fixed in PR #208