Added Subtract Method

Subtract has been added to the polynomial class. Tests updated to match.
Division

Implementing division for polynomials. No tests yet.
More Division Work

Going to have to leave it here for tonight due to coursework
constraints.
Finishing Basic Polynomial Work

Finished (and fixed!) the division function. Added tests for
everythang. Fixed a bug in the multiply function which would fail on
some inputs, such as (x + a)(x - a).
PHP 5.4 Bugfix

“Can't use method return value in write context” - putting a function
into empty breaks because of course it does.
Another 5.4 Bugfix

Should’ve spotted this one. Oops! Also added PHP5.6 to the test list.
Tab in YAML

Far too many hurried commits.
Embarrassment.

Now I’m hurrying too much. This should (pleasepleaseplease) fix
everything.
Merge pull request #3 from tomalom/dev-master

Glad that's all sorted.