Improve processing interception (#266).

As part of this, there is a big change in that request processing no longer happens concurrently for a given request. We do not begin background operations until the foreground processing stack has unwound (#265).
Use local variable.
Improved docs on ProcessingInterceptor.
Update pac4j tests to run without a browser (#257)
Make ProcessingInterceptor test slightly more lenient

We got a Travis CI failure because the backgroundTime was exactly 100.
https://travis-ci.org/ratpack/ratpack/builds/20734648
Fix typo in Registry javadoc
Update ratpack-pac4j to store the user profile in the request registry (#257)
Use internal package for non-public parts of ratpack-pac4j implementation (#257)
Add manual content for Coda Hale Metrics module (#206)
First draft of heroku manual chapter (#248).
Use localrepo when testing Gradle snippets (#248)
Enable testing of snippet in heroku chapter (#248)
Update heroku manual chapter to use the default gradle buildpack (#248)
Minor tweaks to the pac4j module code examples.
Make each sentence start on a new line.

Makes the diffs easier to read.
Minor formatting.
Some work on the Heroku deployment documentation.
Implement a chapter numbering policy to make it easier to insert new chapters.

We may end up adding more granularity as time goes on.
Implicitly respect the PORT environment variable to make Heroku/CloudFoundry deployment more convenient.

Closes #239.
Change heroku chapter title.