Handle proposal cutoff dates.

Use timecop to freeze time in tests. Do not allow users to create, view new,
view edit or update proposals after the cutoff date.
Close proposals on March 18th at 12:00AM EST
Update the CutoffDate application helper method to display date and time.
Change copy when proposals are closed.
Create past_cutoff_date? method.
Add hear back date to the welcome closed copy.
Merge pull request #34 from burlingtonruby/proposal_end_date

Handle proposal cutoff dates.