Add flake8 testing to travis
Remove init imports

This commit removes imports from __init__.py files. Now, all of the init
files are empty. Instead, all of the code has been updated to use module
imports, e.g. `from ..models.coordinator import Coordinator`. While this
makes the imports more verbose, it fixes the flake8 warnings and makes
the code more readable (imho).
Fix travis
Use old travis nosetest script
Add verbose to flake8
Add makefile; add to travis
Update readme
Standardize packet API

Packet's now have a `data` attribute instead of `samples`, `rf_data`,
etc. The tests have been refactored accordingly.
Remove echo from Makefile
Update sensor_reading to use packet.data
Refactor `to_dict` into util

`to_dict` now accepts arbitrary objects. Add repr and str support for
sensor readings and packets.