Add support for explicit exception chaining

A la PEP 3134
Tests for explicit exception chaining

This also breaks out the PY3 only tests into their own file.  We need to do this because raise from is a syntax error in PY2, so we can't rely on the previous hack of catching a HyCompileError - it would compile fine through Hy and then be a syntax error in Python.
raise hy compile error on Py2.x for raise from
revert yield-from test

Yield-from was introduced in 3.3, so we have to catch the compile error that's raise in 3.2. :(
(force Travis CI to rebuild)
Update Dockerfile to use the "python:3" base image, which simplifies things
Merge pull request #621 from tianon/docker-from-python

Update Dockerfile to use the "python:3" base image, which simplifies things
Export filterfalse/input/zip_longest
Remove 'remove' (duplicate of 'filterfalse')
*cough*
Add a simple .mailmap file
Added read and tests
Typo fix s/reprsntation/representation
Merge pull request #633 from theanalyst/fix/typos

Typo fix s/reprsntation/representation
Renamed stdin -> from-file and removed apply from tests + docs
Implement interleave and interpose
Merge branch 'master' into pr/628
Merge branch 'master' into pr/635
Merge branch 'master' into pr/584
Fix RST code-block rendering for (read) examples

Also removed trailing whitespace and added a blank line to separate the discrete examples