Honor $WEB_CONCURRENCY environment variable.

Allows for changing concurrency configuration without changing
application code. This is an environment variable that other
communities (e.g. Rails) are starting to follow, and have enjoyed for a
little while now.

Have discussed with /cc @benoitc in the past and he was +1
Use $WEB_CONCURRENCY environment variable for default workers value
Add documentation for WEB_CONCURRENCY
Merge pull request #686 from benoitc/web-concurrency

Honor $WEB_CONCURRENCY environment variable.