Add parser and ast subprojects, SBT clean up.

In order to publish the Jawn parser and AST separately, we'll need
separate SBT projects. This commit adds these projects, but doesn't
yet move any code over. Things seem to be working with these changes.
Move Jawn AST into ast subproject.

This commit moves Value.scala and makes the other changes necessary
to get the projects all compiling. There is still some more clean up
to be done, but at this point the Jawn AST is not leaking into the
parser implementation.
Move the parser source out of core into parser SBT project.
Set projects up correctly for possible publishing.
Set up bintray SBT plugin.
Better comments and some fixes for parser.

Releasing jawn-parser will require producing good documentation on
how one is expected to use (and extend) it. The first step to that
goal is to produce better method comments.

The next step is to produce some good markdown in the README.
Add Try, and update benchmarks

This commit simplifies Jawn's JParser interface. We are replacing
Either[Exception, J] with Try[J], which is more in line with what
folks in Scala 2.10 are doing.

It also introduces another parser to the benchmarks:
scalastuff/json-parser.