Use opam pin for development
opam: only specify major opam version

Doing otherwise trips up earlier versions of opam, even though the file
is compatible with them.
opam: update README
opam: update travis build matrix
opam: remove unnecessary line in _tags
Merge pull request #54 from frenetic-lang/opam

Use opam pin for development
Added core and sexplib to oasis dependencies
Use Jane Street's Core as Standard Library

- Use sexp conversions for most types
- Use Core's collection types (Map, Set, Array and Hashtbl)
- Change function calls as needed for the Core equivalents
Added sexplib and comparelib as dependencies
Add sexp and compare to all exposed types
Core: fix function return type warning
Core: Updated travis configuration