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