Updated homepage, installation, and usage docs

[ellbosch]

Tag: #310

Note: Git's diff is largely ineffective for this PR. I've provided 'before and after' links to showcase doc changes.

This PR introduces sweeping changes to our docs in preparation for GA. Changes include:

• New README.md for homepage (before and after)
  • Expanded description for features
  • New 'Quick Start' section to get users up and running faster
  • Updated python badge
• New README.md for docs/installation (before and after), replacing old installation_guide.md
• Improved Linux installation docs (before and after)
  • Expanded Ubuntu section per release
  • Removed repetitious language around direct downloads
• Improved usage docs (before and after): added config file section, other minor changes
• Brings documentation back to our PyPI profile site

[ellbosch]

@pensivebrian sorry for muddling in some minor pep8 related fixes, I can separate those changes into a separate PR if you'd prefer that.