When users follow @blocktogether, follow back.

Allows a two-way feedback channel via DM.
Merge pull request #50 from jsha/follow-back

When users follow @blocktogether, follow back.
Display screen names on the Actions page.

Also add a necessary association from Actions to TwitterUsers.
Attach a cause and cause_id to Actions.
Add robots.txt.
Merge pull request #51 from jsha/actions-by-screen-name

Display screen names on the Actions page.
Merge pull request #52 from jsha/actions-have-cause

Attach a cause and cause_id to Actions.
Miscellaneous fixes.

Update-users now stores looked-up users one more (broke this when refactoring).
Log On (vs Sign Up) now sets cookies and redirects correctly.
Add btn-primary class to buttons.
Add "(beta)" in homepage header.
Log friendships correctly.
Correctly find new log-on-form
Improve enqueueBlocks function signature.
Log off deactivated users automatically.

Also remove unnecessary isAuthenticated function.
Update testing instructions to reflect latest changes.
Merge pull request #53 from jsha/misc-fixes

Misc fixes