FIX/COSMIT helper to identify target types

* Fixes bugs related to simply checking type(..) equality
* Refactors metrics and preprocessing
* Invalidates mix of sequences of sequences and label indicators in
  metrics
* Invalidates row vectors in metrics
* Fixes bug where single-class target led to incorrect F-score
FIX cannot use set notation for Py2.6
FIX need explicit dtype for array of sequences in numpy 1.3
COSMIT remove redundant target size check

This error is already being raised by check_arrays
FIX numpy 1.3 has no float16; use float32
FIX/TST np.squeeze in numpy1.3 fails with array of sequences
FIX numpy 1.3 throws error with array of arrays
FIX make Pipeline methods properties as per #1805
TST test delegated ducktyping of metaestimators
TST extra tests, pep8, comment
Fix issues in test