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.
Merge pull request #2848 from richdougherty/play-io

Reduce size of PlayIO API