RF: better tests of current axis finding algorithm

I didn't understand it myself, so working through with tests
RF: make fix0=True default for axis routines

I believe fix0 is benign and there are many TR=0 images out there; make
fix0=True the default so people don't often have to futz with the affine
in TR=0 case.
Merge pull request #244 from matthew-brett/refactor-axis-finding

Refactor axis finding

After using it for a bit, it seemed to me that we should always fix the very common TR = 0 axis problem (at least by default). Changed the code and tests to reflect this.