Add EditorConfig file

This does two things:

* Adds a `.editorconfig` file specifying the indentation (useful for the Travis and Tox files in particular), whitespace trimming options, and some other file-specific settings.
* Removes trailing whitespaces from the README.rst file

I'm biased because I was the original author of this format.  I'm particularly fond of explicit project-specific settings files (`.coveragerc`, `tox.ini` or `setup.cfg`, `.editorconfig`, `.jshintrc`, etc.) so I'm probably also biased for that reason also.

These files don't often make it into Python projects mostly because indentation is the most important issue it handles and Pythonistas all all tend to use 4 spaces already.