Spell WordPress correctly (with a capital P)

WordPress is spelt with both an uppercase W and P. This pull request fixes up a few instances of WordPress being spelt with a lowercase P.

Many in the WordPress community are quite protective of this camel casing of the name. There is even a core function to [make sure it is spelt correctly in post content](http://codex.wordpress.org/Function_Reference/capital_P_dangit). So it's important to make sure it's spelt correctly in plugins readmes etc.

Great plugin by the way. :)