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.
NF: results_postproc_fx for Searchlight

To facilitate parallel postprocessing of the blocks of results to
please Swaroop
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
DOC: spacing
ENH: put a newline at start of searchlight status when in debug mode
RF: renamed some methods
ENH: added unit tests
RF: better check whether external exists
DOC: minor
BF: use new get_sphere_left_right_mapping
BF: unit test uses new "reposition_hemisphere_pairs"
ENH: fix resolution