emacs: fix handling of company's "doc-buffer" command.

Related to #238: hitting <F1> now works "as expected".
emacs: fix handling of company's "location" command.

Fix #238
locate: add cache on cmt files
lexer: handle one more corner-case in reconstruct_identifier
Install merlin-iedit when using opam
Merge pull request #274 from jpdeplaix/install

Install merlin-iedit when using opam
locate: better control over file switching
Resulting in more coherent error messages in case of errors, and less "false
positive".
emacs: display in lighter when there are error in the buffers
Even the ones without a position, which were previously ignored. Examples
include: cmi errors.
emacs: display a message when type-enclosing doesn't return anything
Merged cmi and cmt caches