Move script to a separate file.

Unable to take advantage of my editor otherwise.
Also one can link to the separate script from userscript definition,
allowing for easier development.
Clean up some JavaScript quirks.

- Turn on strict mode.
- Add missing semicolon.
- Eliminate check for minutes != 0 (dividing 0 is a safe operation).
- Eliminate unused `result` variable.
- Remove array initialization, it's never used.
- Replace !=null test with simple truthyness check.

Mostly because I'm customed to running JSHint while development
to report me of possible problems, but it's of no use when the
whole code shows up underlined.
Rename accum() to parseToMinutes() to aid my understanding.

Also just check for the truthyness of `time`.
Write the sum into new row in footer.
Calculate both of the estimate columns.
Round the summary row to minutes/hours/days as appropriate.
Regenerate footer when #issuetable is recreated.

Monitor page content area for DOM changes.  When the table is
re-created re-calculate the footer.

This means the footer gets conveniently updated every time the
issues filter is changed.
Add weeks also to summed estimate formatting.

Gives IMHO better overview when summing up lots of issues.
Merge pull request #2 from nene/master

Alternative implementation of adding sums to last row of a table.