Merge pull request #266 from paultag/paultag/feature/yield-from

Paultag/feature/yield from
Fixing blank line Flake8 failures
Add flake8 to Travis-CI to prevent these from coming back

Added new travis make target to skip flake8 on pypy but run it on all others
Removed duplicate flake8 since it has moved into requirements.txt
fix def / setv example
Merge pull request #282 from tuturto/doc

fix def / setv example
Merge branch 'master' into pr/280
Macro if-python2 for compile-time choice between Python 2 and Python 3 code branches
Add zero? predicate to check if an object is zero
Merge branch 'master' into pr/287
Implement gensym

 I'm not exactly sure if this is the best way to implement it,
 but I've gone and matched this implementation to the interface
 shown in:

   http://clhs.lisp.se/Body/f_gensym.htm

 Close #278
Move gensym prefix to a UTF reserved char

 Also add tests for gensym behavior.