Call sentry only when it was set up.

Fixes issue 1727.
Send traceback to log if Sentry not set up.
Trim trailing whitespace
Fix regression post-gittip.db; #1742

The hack gets utterer.
Merge pull request #1750 from gittip/issue-1742

fix one more regression w/ gittip.db
Bump version to 10.1.51
Bump version to 10.1.51-dev
Disallow `/on/*` in robots.txt
Merge pull request #1755 from gittip/issue-1753

Disallow `/on/*` in robots.txt
Merge pull request #1730 from gittip/issue-1727

Call sentry only when it was set up.
Add decorator log_cursor - prints all sql to stdout.
Control if the homepage updater thread gets run.
Control the output of metrics to stdout.
Limit the number of transfers to 5 to speed up the test.
Add pytest-cache.

Adds the option to rerun only tests that failed in the last run.
Merge pull request #1756 from gittip/nicer-dev

Nicer dev environment
#1756: add gitignore for pytest-cache