TST : fail sensibly on py3k

Fail sensible in py3k for c-extension related tests.

Use the `known_fail_if` decorator to fail the ctrans related
tests if we are under python 3
MNT : simplified process_to_q test

Simplified code in test to allow easy extension to testing more points.
TST : Added more tests to test_recip

Uses test that has one point in the center of each grid.
Now checks that all points are in grid, the occupancy is correct, and
that intensity array is exactly equal.
TST : added second test to test error

Added second test of gridder
ENH : made exceptions have useful messages

Now tell the user what the ValueError is (shape of input data)
DOC : corrected documentation of gridder

the error value is actually the standard error, not the standard
deviation.

Made other docs more verbose
MNT : renamed returned values in process_to_q

grid_data -> grid_mean
grid_std -> grid_error
grid_out -> n_out_of_bounds
Merge pull request #86 from tacaswell/recip_test

Recip test