Remove install-time dependency on Jinja2

As discussed at length in #50, an install-time dependency on Jinja2,
especially when locked to a patch version of the package, causes more
problems than it solves.

This commit moves the Jinja2 dependency to a setuptools "extra", so
that `pip install honcho` no longer pulls it in, but it is still
possible to automatically install with

    pip install honcho[export]

As this is a breaking change (for people who were relying on `pip
install honcho; honcho export ...`) it will require a minor version
bump.

Fixes #50.
Use honcho version string in docs
Add documentation about PYTHONUNBUFFERED

Fixes #45.
Deal with crazy non-POSIX shells

Heroku's shell violates POSIX by having neither USER nor LOGNAME
environment variables exported. Catch this and provide a user-friendly
error message rather than a KeyError.

Fixes #33.
Allow to start more than one specified processes
Refactor and clean up integration tests
Add test for "honcho start" with multiple args
Bump version -> 0.5.0
Fix for UnicodeDecodeError
Python3 fixes
Merge pull request #54 from jeethu/unicode_error_fix

UnicodeDecodeError raised if child process writes invalid utf8 characters to stdout
Update "Differences to Foreman"

It would have been helpful to me to have a better sense of Honcho's relationship to Foreman before wading into the project. This commit works the comment at https://github.com/nickstenning/honcho/issues/55#issuecomment-34163304 into the "Differences to Foreman" section of the documentation.
add new sections to docs and installation

sections added:

* Contributing
* Credits
pep8
add missing requirements file for contributors
extra environments for tox
use tox environments instead of travix matrix

also use coveralls
add Makefile for developer convenience
add coverage configuration file
use built-in coverage mechanism. ref e89c1c5