Lots of changes, some of them breaking, but overall just providing nicer abstractions over the underlying pandas and igraph stuff.
* Introduced `TextnetBase` and `ProjectedTextnet` classes, and made `Textnet` a descendant of the former.
* Improved code modularity to make it easier to add features.
* `Corpus` is now based on a Series rather than a DataFrame.
* Added methods for creating an instance of `Corpus`: `from_df`, `from_csv`, `from_sql`.
* Expanded and improved documentation.
* Added bibliography to documentation using a Sphinx bibtex plugin.
* A first contributor!