Release 1.4.0

1.4.0 (2014-08-10):
Comprehensive addition of bounds checking to all cstruct operations
(from @pqwy in #33).  The major changes are:
* Disallow negative indexing with all cstruct accessors.
* Disallow negative `sub` and `shift` operations.
* Make sure `of_bigarray` cannot create invalid `cstruct` values.