Update guides

[cprussin]

Go through guides and update based on changes since the last release, and double check that all examples are still accurate

• Add notes on Body typeclass, along with different instances (String, Buffer, Stream.Readable, etc)
• Add notes on default headers (Content-Length, etc) and mechanism for overriding
• Add some notes on common path matching techniques and tools (things like array pattern matching, dealing with case sensitivity, purescript-routing, etc)