RF: Consolidate attribute assigning in Searchlight's result_fx

With different result_fx implementations that shape and content of
attributes can be totally different, so all this should consistent
with the actual result_fx
ENH: Extend pass_attr feature to support additional use cases

Like:
- renaming
- axes swapping
- explicit target attribute collections

fully backward compatible
RF: Split _postcall in Measures

This is necessary to allow for picking up the 'null_prob' CA in Node's
pass_attr facility.
Merge remote-tracking branch 'gh-hanke/master'

* gh-hanke/master:
  RF: Split _postcall in Measures
  ENH: Extend pass_attr feature to support additional use cases
  RF: Consolidate attribute assigning in Searchlight's result_fx

Conflicts:
	mvpa2/tests/test_datameasure.py
RF: just for consistency move michael's test inside the suite
BF: fixing up our test for cherry-picked changes from Michael