Split project into several submodules:

- xproc-engine-api: The XProcEngine interface. Each implementations of
    this interface now has its own module, and are made available either
    through an OSGi service or through Java's "service provider loading"
    mechanism.
- xproc-engine-calabash: The implementation that wraps for Calabash.
- xprocspec-runner: The code that executes XProcSpec tests is put in its
    own module in order to make it reusable outside of the context of
    this Maven plugin.
- xproc-maven-plugin: The plugin.

The DAISY Pipeline 2 related stuff was removed for now because it has
unreleased dependencies.