Updated unit tests
Fixed incorrect documentation
DSL and Implicits now traits, added base extractor

morph.ast.DSL and morph.ast.Implicits changed to traits so they can be
mixed in with classes. There are also companion objects so that the
implicit conversions can be imported into scope without mixing in the
trait. Importing the companion objects' methods also makes it easier to
test things in the REPL.

Also fixed comment style.
Added filter and is[something] methods to DSL
Set up build.sbt to auto-import morph code in REPL

To make testing in the REPL easier, configured SBT to auto-import all
morph classes, objects, and implicit conversions into the current scope
when the REPL is started. That includes the following imports:

* morph.ast._
* morph.ast.Implicits
* morph.ast.DSL._
* morph.parser._
* morph.parser.Interface._
Updated sbt config with instructions on build dep