FF: various fixes to degFlat and degFlatPos unit conversions picked up by new tests

- make sure that pos and vertices are numpy arrays
- make sure that returned vertices have datatype 'd' (not 'f') or shapes go wrong
- cleaned up code for using these units with Text and SimpleImage, which had
additional references to unit code in their bodies
TESTS: Added tests for the new window units, 'degFlat', 'degFlatPos'

Also fixed minor discrepency in returned fit params for naka-rushton
TESTS: relaxed tolerance on fit naka-rushton

(32bit mac and 64bit linux come to slightly different optimum)
DOCS: better info about params for functionFromStaircase
TESTS: is aperture now working with Travis servers?
TESTS: removing xfail of rating scale for Travis (to see if now working)

also removed the word 'fail' from the linux code that changes thread priority
(to make it easier to find genuine test failurs on travis)
BF: array size checking for paramaters should check v.shape[-1]==3
not len(v)==3

If the param is for an element array then it should be [nElems,3] and this
was failing the test.
DOCS: changelog
ENH: added rusocsci package to the Standalone distribution
BF: relax requirements on wx version
DOCS: updated changelog and index.rst
Merge branch 'master' of git://github.com/psychopy/psychopy