added __str__ to classes in vectorstore.py
added safety check in VectorBackedSelectKBest
Fixed a couple of tests that failed because of refactorings done earlier today. That involved indenting a bunch of test methods back into their class and changing the default value of a ensure_vectors_exists in VectorBackedSelectKBest so that when a pipeline creates this object with no parameters the default values do not conflict each other
Expanded (to bigrams) and cleaned up tests for feature selectors, which also sort of blackbox the whole process from text through vectorisation (with a mock vector source) and feature selection
optimised calls to logging so that parameters to disabled logging level are not calculated