Add support for multiple predicates

We're supposed to be able to handle multiple predicates, but so far our
code only works with one.

src/range.rs
------------

Let's split the parse function up.  Most of what we already had will be
called `parse_predicate`.  It works on one predicate at a time.

We'll use `parse` to split apart the string on commas and send each
portion to `parse_predicate`. A quick bit of error checking verifies
that we have at least one predicate, then we can return our
`VersionReq`.
This tag has no release notes.