Implement parsing
Add atom level keywords

Change keyword generation to calling a class function
that builds Keyword objects from a list of strings.
Support python expression generation pt. 1

Generate conditional expressions for single conditions.
Adding support for the "infix" boolean combinations
comes next.
Add tests for parsing

Right now, they mostly test conversion into an
unambiguous string. Once I get the infix operators
generating python code, I will add tests for that to
each case.
Add aliasing for boolean (infix) operators
Add expression generation for infix operators

And tests. And renamed tests: test_select_blah -> test_blah
because the file is test_selection so it was redundant
Refactor mdtraj_expression -> mdtraj_condition
Add test for syntax parsing of "within _ of _"

I don't know how I'm going to implement this.
Add select method to topology
Appropriately quote string values
Add license
Start building custom extension for documentation

Clean up license, manifest, readme (which I thought
I deleted during the doc cleanup)
Small refactor in support of doc building

- Change top_name to be the name of the class in topology.py
- Get rid of keywords list. Instead use keyword_aliases.keys()
- Make the target field the actual name of the property
- Change top_name et al to be class methods
Auto-generate atom selection docs
Move element selection to new class

- Add test
- Add docstrings to element properties
Add some documentation
Templates for properties

- Add some not-implemented properties
- Make existing docstrings better
Organize imports
Construct lambda filters

Haven't implemented "range" yet
Cleanup