2.0.0~beta4

This beta brings many small fixes, some polish to the CLI, and most importantly a built-in solver, so it isn't needed to care about having aspcud installed anymore.

More detailed log:

• Building with OCaml < 4.02.3 is no longer supported
• Support compilation with a built-in mccs solver (removing run-time dependency to aspcud). Integrated half-working "heuristics" dropped.
• Remove jsonm and transitive uutf dependency.
• Switch build systems to jbuilder, including lib-ext support
• Allow repeated and or'd arguments to the list command
• Many code and build system portability fixes (Windows, BSD)
• Add switch export --full to include package definitions
• Add announce: and stamp: fields to repositories (repo files)
• Add a global cache of git objects (greatly speeding up multiple cloning)
• Allow opam pin URL without a package name
• Many error handling, messages and corner case fixes
• Updated the versions of dependencies
• Removed the unused features: field, and proposal for a new syntax
• More informative exit codes, and documentation thereof