TST: add tests for centrfrq and scal2frq.
TST: add test for integral on orthogonal wavelet, intwave().
MAINT: minor cleanups in functions.py
Merge branch 'pr/37' into master.

See https://github.com/rgommers/pywt/pull/37
TST: adding doctests for thresholding as unit tests.
TST: add some tests with 2-D arrays for thresholding functions.
Merge branch 'pull-39' into master.

See https://github.com/rgommers/pywt/pull/39
Merge remote-tracking branch 'upstream/master'