Unit tests for gridster

Hi there,

People seem to have lots of issues with gridster, and most of the time try to explain with pictures or videos. I propose to require new issues to have a failing unit test in the future.

I've written some unit tests myself to illustrate some bugs I've found in gridster, and hope they can be used as a starting point for other people. Fixes for the failing test cases are included in this pull request as well.

I've added a `max_rows`-option to make testing easier (otherwise it's hard to control the number of rows/cols).