Updated utils.timer to use modeling, related doc, and tests. Updated conesearch timer to reflect changes in utils.timer. Fixed timing related tests in conesearch.
Moved six import to the top as suggested by @mdboom
do_fit() method now accept any astropy.modeling model and fitter, as suggested by @eteq
Fixed do_fit() method documentation as suggested by @eteq
Merge pull request #1896 from pllim/timer-fitter

Updated utils.timer to use modeling, fixed related conesearch tests