Add `with sexp` serializers to all the Ipaddr types

The representation of IPv4/6 addresses is specialized to the same
as the human-readable string, so the internal representation is not
exposed.  The other types (such as Prefix) are represented using
the default sexp conversion functions for variants and tuples.
Add test cases for Sexplib; mainly checking bidirectional conversion
Add OASIS autogen for the sexp accessor changes
Travis: update dependencies
expose V4.t with sexp, too
Clarify sexp parse error messages
remove redundant subexpression
Add sexp for Macaddr, too
Add test coverage for V4/V6/Prefix sexp string roundtrips
Merge pull request #31 from mirage/sexplib

Complete sexp serializer patch