Make some minor changes for consistency with the rest of the code.

* Rewrap docstrings to 79 columns, and reformat them as Sphinx syntax.
* Move docstring above imports.
* Indent grammar, and wrap it to 79 columns.
* Compact a few returns.
* Remove the unneeded encoding declaration on an empty __init__.
Shorten and simplify a bit.

* Instantiate the Grammar at import time.
* Make a >0 test more idiomatic.
* Take the word "Node" out of visitors for brevity.
* Replace the StringExpressionLanguage class with 2 simple functions.
Use nose, simplifying tests.