Performance enhancement and usability improvement

First off: The current project had a failed unit-test here, so I decided to leave it that way... if I build the plugin with skipping the tests the plugin works fine. Haven't looked into that, just made the fixes I wanted :-)

The improvements:

1) maven.user.settings: One of my colleagues has custom settings.xml files with custom .m2 repo directories. He wanted to be able to use the parameter maven.user.settings to define the settings.xml to use. I've added this code.

2) Performance! With our upgrade to the latest Fitnesse build (20130530) we noticed a speed increase in running the tests but a huge timeout before the tests start running. After profiling it turns out that the Maven plugin is building again and again for each testpage (?). This resulted in a white page, without any tests being run for up to 2-3 minutes. I've created a little caching mechanism in the plugin so particular builds are only done just once, the rest is retrieved from a HashMap. This greatly increase the speed running our entire suite.

(The only problem is you don't see any speed increase because Fitnesse starts measuring after parsing all the pages and loading the plugin multiple times)