add Kahan summation (numerically-accurate sum of floats) to List,Array,Enum

This change is inspired by
  http://re-factor.blogspot.fr/2013/10/kahan-summation.html
itself coming from
  http://adereth.github.io/blog/2013/10/10/add-it-up/

I discovered in the process that Kahan Summation was already
implemented in Batteries, for Enum. In this case, the existing
overhead of accessing the next element apparently made it an option
for the 'fsum' function already.
some benchmarks of Kahan summation
Add BatteriesExceptionless
Include Batteries in BatteriesExceptionless
Merge pull request #460 from jpdeplaix/master

OCamlbuild: Use batteries.mllib instead of batteries.ml while trying to build batteries.{cma,cmxa}
Merge pull request #459 from gasche/master

Kahan Summation in `Array`, `List` and `Enum`
Merge pull request #461 from jpdeplaix/exceptionless

Add BatteriesExceptionless
added BatDeque.eq function to compare Deques by content
basic .merlin file for merlin users
Merge pull request #462 from c-cube/master

basic .merlin file for merlin users
batSeq.{of_list, equal} and some tests
levenshtein distance on strings
Merge pull request #463 from c-cube/deque-equal

added BatDeque.eq function to compare Deques by content
Merge pull request #464 from c-cube/seq-fixes

batSeq.{of_list, equal} and some tests
small optimization for BatString.edit_distance
Merge pull request #465 from c-cube/levenshtein

levenshtein distance on strings
Add List.{is_contained_in,image}