Add yield from via macro magic.

 This will let us use (basic) yield from behavior from Python 2. This
 isn't complete, and is low-hanging fruit for others willing to hack
 on hy.

 I've also changed the macrosystem to allow for proper bootstrapping.
 This is similar to how it's done elsewhere in the codebase (stdlib
 stuff).
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