Fixed mistake in POD of Solver

At some point in the past the Solver had a parameter called operations,
but that parameter is now called nodes. The POD is now updated to
reflect this.
Fixed file name sanity check.

The file name sanity check in the to_png() and to_dot() methods on the
Solver object was incorrectly only allowing lower-cased file names.
Added tests for the Operation and Solver modules.

Added tests which cover the basic functionality of the Solver and
Operation modules.

Added dependencies to dist.ini to account for additional dependencies
used in the Solver tests for testing the to_png() and to_dot() methods.
Updated Changes and dist.ini for version 0.03.

Release early, release often.