Give user-assigned test runners priority

Concerns #9
Add test and handle no nearest test

Copied the nose test suite into the pytest suite
and alter to match calling conventions of pytest.
Implement check for executable and variable

The test checks for the correct executable, or
the value of g:test#python#runner which must be
set by the user, along with the file name check.

See #9
Update readme to include choosing python runner
Merge pull request #9 from mindriot101/add-pytest

Add pytest runner
Fix Python

Concerns #9
Add the "make" strategy
Minor readme modification