Add missing documentation so that build test works

Improve documentation to specify how to create a SNAPSHOT build in the local repository.
Remove RouterIO and replace with commons-io

This gives Play a build-time dependency on commons-io, but not
a runtime dependency.
Merge pull request #2842 from richdougherty/router-io

Remove RouterIO and replace with commons-io
Case-insensitive handling of response headers
Merge pull request #2829 from willthames/patch-1

Add missing documentation so that build test works
Reduce size of PlayIO API

* Inline some methods into the Files API
* Always close when reading a stream
* Didn't use scala.io.Source because it doesn't really support reading
  binary data and because it looks slow, which wouldn't be great for some
  use cases, e.g. reading assets or when a user uses Files.readFile.
Build script improvements

* Detects Java version and doesn't add MaxPermSize argument if 1.8 or
  later (copied from Activator)
* Support for a debug parameter (similar to old play script)
Merge pull request #2851 from jroper/build-script

Build script improvements
Merge pull request #2848 from richdougherty/play-io

Reduce size of PlayIO API
Merge pull request #2815 from gmethvin/case_insensitive_headers

Case-insensitive handling of response headers