Format cycles from simple_cycles w/o last node

Cycles in networkx are represented by a list of nodes.  But
simple_cycles() repeated the last node of the cycle.  This removes
that repetition to bring simple_cycles in line with other cycle
routines.  Related discussion in #889, #890.

Also corrected some doctests and updated tests for this change.
Merge pull request #897 from dschult/fix_cycles

Format cycles from simple_cycles w/o last node