* Rename and move CordaPluginRegistry to reflect its real purpose now.
* Docs: docsite improvements
* Remove discussion of webserver from 'writing a cordapp' page.
* Fixup some flow docs.
* Add a couple more package descriptions.
* Review comments - always apply default whitelist and no longer load it via ServiceLoader
* Added wording about renaming services resource file
This improves the Java API and makes it more idiomatic. The methods
were not moved to be static methods of the relevant types in all cases
due to a bad interaction with a Kotlin auto-completion bug, and because
static methods on interfaces are new in Java 8 and Kotlin is not yet
emitting Java 8 bytecode.
Also, introduce a packages.md file so packages can be documented.