Added more tests for Bitvector, Sequence, and Tree

- fixed minor issues with Sequence module
  - incorrect comments
  - added type 'elt' for better documentation
  - 'safe' flag (to be expanded) for error bound checking
  - added better exceptions
  - removed/commented unused functions
  - bug in concat
updated opam template
comments, upkeep
Merge ../Phylocaml
update readme