Merge pull request #141 from pveber/master

Sam: a constructor for optional field
Sam: a slightly more precise typing of optional field values
Sam: remove unused opt_field_A_re

No longer used due to db4644a847af610738704c89b3a18a3ed2b24526.
simplify opam pinning for developers

No longer need to add special repo prior to pinning biocaml. Just do
`opam pin -k path biocaml ~/path/to/biocaml`.
opam: update for opam 1.2 features

Separate out install target. Add bug-reports and license field. Add
build constraint to ocamlfind and omake.
opam: add version constraint on camlzip

We require the `zip` findlib package, which is not provided by camlzip
1.04 (as mentioned in #144). We could instead require `camlzip`, which
would make us compatible with all versions of camlzip. However, it
seems 1.05 is deprecating the `camlzip` findlib package name in favor
of `zip` (because camlzip is defined as an alias to zip, the library
file is zip.cma). Thus, might as well go with that.
bam_alt: parsing of optional fields
bam_alt: a tiny bit of documentation