Improve design and test coverage of export package

This PR:

- Moves responsibility for I/O back into `honcho.command` rather than `honcho.export`.
- Changes exporters so that `#export()` takes a list of `honcho.environ.ProcessParams` objects and optional template context. This means that concurrency, process naming, and port assignment is now much more likely to be consistent between `honcho start` and `honcho export`.
- Improves test coverage of the `honcho.export` package.
- Removes a couple more fragile export integration tests.

Maybe replace `print(e)` with `log.exception('Error in _mkdir')` (see https://docs.python.org/2/library/logging.html#logging.Logger.exception)?