Merge pull request #18 from vjandrea/2.0

Added $columns parameter to tree() method and added new filteredTree() method.
Merge pull request #19 from vjandrea/2.0

Fixes an ErrorException - Undefined offset: 0 in Collection.php:16
removed unnecessary space between method definition and curly brace
Merge branch '2.0' of https://github.com/franzose/ClosureTable into 2.0
created blank testFiltetedTree method to further implementation
added  argument to retrieval methods
corrected params in phpdoc of filteredTree() and buildTreeQuery() methods
fixed wrong return type in phpdoc of filteredTree() method
Merge pull request #1 from franzose/2.0

2.0