Add dc.js line graph for average pull request ttl

Prior to this commit, the only graphs on the dashboard were the two
bar graphs displaying number of pull requests closed over time
and per repository.

Now a new graph has been added. It displays average lifetime (in
days) of pull requests over time, where the "lifetime" is the time
from open to close.
Add graph of pull requests merged per week

Prior to this commit, the number of pull requests merged was only
being displayed on a monthly scale. Now another graph has been
added which shows how many were merged per week.
Add basic usage instructions in README.md