Allow pra to recover when fetching raises errors

I did this so that pra will not crash when fetching pull requests
fails (e.g. due to http timeout, missing vpn connection, laptop was
closed for 5 minutes, etc). If there is an error during fetching, the
short message will be printed to the screen and the full error with
stack trace will be logged to ~/.pra.errors.log, and pra will keep
running and retry later.
Let pull sources fail without impacting the others

I did this so if a fetch from a pull source fails due to a problem
that doesn't affect the other sources, we can still display the pull
requests that can be fetched successfully. I also changed the error
display to print a generic message telling the user where to look to
find detailed errors.
Merge branch 'dont_crash_on_failed_fetch'
Added ChangeLog.md entries for new features.
Bumped to version v1.0.0
Made .ruby-version rbenv & RVM compatible.

I did this so that developers that are using rbenv or RVM could just as easily
work on this project.
Add assignee column

To visually determine if a pull request is currently assigned
Merge pull request #11 from reachlocal/add_assignee

Add assignee column
Updated the ChangeLog.md with assignee column.

I did this so that people could see that the Assignee column was added as a
feature.