Create core project with non-jvm-only dependencies

[johnynek]

I would like to be able to run princess on scalajs (see issue #34 ).

This PR attempts to move all non-JVM code into a core package, leaving the current APIs that require the JVM in the src directory.

I don't think this is ready to merge, but it is here to see if you are open to a refactor along these lines.

[johnynek]

obviously this changes semantics, but if we were to pursue this we could implement a version of this file for the JVM and one for scala JS and implement the intent correctly.

[johnynek]

obviously this isn't safe in a threaded environment, but if the approach is interesting to you we could implement a jvm version of this file and js version of this file and select the correct one at build time.

[pruemmer]

This looks like a reasonable approach! Two thoughts:

• There are some dependencies of theory solvers on the SMT-LIB parser AST, though the interface SMTParseableTheory. This probably means that the actual AST has to be include in the core project.
• The main place where threads are used is the SimpleAPI. It would be possible (probably) to refactor this class to have multiple back-ends, both single-threaded and multi-threaded ones.