BUGFIX: fix 1st order CRFB handling

For when the user tries to synth a 1st order CRFB modulator -
which happens to be the default form, so it seems worthwhile to
handle this gracefully.
FIX: xlim can be a 1d array.
BUGFIX: ensure no zero-length arrays are ever returned.
Finished adding tests for the _d* functions.
Merge pull request #37 from schodge/d_tests

TESTS: Add tests for the _d* functions.
BUGFIX: fix 1st order CRFB handling

For when the user tries to synth a 1st order CRFB modulator -
which happens to be the default form, so it seems worthwhile to
handle this gracefully.
FIX: xlim can be a 1d array.
BUGFIX: ensure no zero-length arrays are ever returned.
TEST: remove tests from _circ_smooth.py.
TEST: remove tests from _clans.py
TEST: remove tests from _DocumentNTF.py
TEST: remove tests from _ds_freq.py
TEST: remove tests from deltasigma/_ds_optzeros.py
TEST: minor docstring change for consistency
TEST: remove tests from _ds_quantize.py
PEP8: fix indentation
TEST: remove tests from deltasigma/_ds_synNTFobj1.py
TEST: add test for f0=0, fix minor c&p error
TEST: remove tests from deltasigma/_dsclansNTF.py
TEST: move tests for delay() to the new location.