Make developer dependencies optional about chatexchange

[banksJeremy]

It doesn't look like there's any specific way of indicating dev dependencies as with Node, so a [dev] extra as implemented in fc8d339 seems like the appropriate approach.

It might be nice if we could have a second, distinct, Travis test build, which tries installing the non-dev dependencies and running a simple example script, but I worry that that's a slippery slope to lots of disorganized testing so I probably won't pursue it.

[banksJeremy]

(Although if we did want to go down that road, I think it might just take something like this (docs):)

matrix:
  include:
    - install: 'pip install .'
    - script: 'python examples/something.py'
  include:
    - install: 'pip install .[dev]'
    - script: 'python -m pytest'

[banksJeremy]

The build of fc8d339 failed on Travis. I am guessing there might be an issue with using extras with a non-editable installation of a local path:

(from .travis.yml, doesn't work, it thinks I want a packaged name ".[dev]")

pip install .[dev]

as opposed to calling

(works locally, as make install-dev-dependencies)

pip install -e .[dev]

or

(well-documented usage)

pip install ChatExchange[dev]

[banksJeremy]

I've filed a ticket about this behaviour at pypa/pip#1780, which should clear up whether this is a bug or expected.