Added 2 interfaces for action expression and condition expression plugins.
Merge pull request #67 from klausi/rules-expression-base

Interfaces for action expression and condition expression plugins.
Removing workarounds for PHPUnit Bug now that we are running on PHPUnit 4.
Refactoring context system overrides out of Rules.
Merge pull request #65 from fubhy/removing-context-system-override

Refactoring context system overrides out of Rules.
Merging ConditionTestBase with RulesTestBase.
Fixing test condition in PathAliasExistsTest.
Initial commit of 'Data is empty' condition with unit test.