Python3 support + small fixes and circular dependency resolving
Merge branch 'master' of github.com:daler/gffutils into 32

Conflicts:
	gffutils/feature.py
	gffutils/interface.py
Merge branch 'master' of github.com:daler/gffutils into 32

Conflicts:
	gffutils/__init__.py
	gffutils/feature.py
fixes to attributes doctest
print function in tests to support py3
always sort merged non-attribute fields

should only be slow in pathological cases -- this will mostly be sorting
a small number of things (like "+", "-", "." for strand field) so
shouldn't be a performance hit
only warn when trying to import pybedtools (which doesn't have py3 support yet)
add option to sort attribute values

this was causing a lot of py3 test failures, apparently i was depending
too much on py2 default order for sets/dicts
sort_attribute_values for Feature
finish fixing tests for py3 support

NOTE: using sqlite3.OptimizedUnicode for db text_factory