Unit tests

This contributes to feature #82 Improve unit test coverage.

When on klaussilveire/gitlist master branch and running phpunit --coverage-html=/tmp/org
there is a coverage of 

## klaussilveira/gitlist/master:
* Lines 72,18%  314/435
* Methods 48.65% 18/37
* Classes 38,46% 5/13

## klaussilveira/gitlist/master, with @covers annotation:
* Lines 37,70%  164/435
* Methods 10.81% 4/37
* Classes 7,69% 1/13

## DannyvdSluijs/gitlist/master, with @covers annotation:
* Lines 51,03%  222/435
* Methods 43,24% 16/37
* Classes 38,46% 5/13

I've done my commit and pull request now, to keep the test current. My plan is improve at a later time, or others could join. Please provide feedback when tests aren't sufficient. Also notice the addition of vfsStream in composer.json.