Add support for spatial and nonspatial Wilkinson formulas

[tdhoffman]

This pull request adds the formula functionality described in #101. It consists of the following parts:

• spreg/formula.py: the main formula implementation (parser and dispatcher), documentation, and doctests
• spreg/__init__.py: add a line which imports the formula interface when spreg is loaded
• notebooks/formula_example.ipynb: a notebook showcasing the features of spreg.from_formula() alongside textual descriptions
• spreg/tests/test_formula.py: unit tests for the spreg.from_formula() function.

Pursuant to this comment on #101, everything in this PR is fully backwards compatible. This is an add-on to the package which sits on top of existing code and provides another way for users to access it. Currently, the function dispatches to all of the spatial regression models. While it does not support regime regression or seemingly unrelated regression, it provides a solid base of code to which these features can be added.

[jGaboardi]

Since #103 supersedes this PR, shall we close it?