ClosureTable 2.1

In this release:

1. Introducing filteredTree method, see #18. You can use it when it's necessary to get a tree, filtered by some condition.
2. Fixed #19 (An exception thrown by \Franzose\ClosureTable\Collection when tree is empty).
3. Retrieval methods now support selecting certain columns by passing array with their names.

Thanks to @vjandrea for the first two improvements.