emacs: Append merlin-ac-source to ac-sources instead of replacing it.

Closes #19.
emacs: moved ac initialization inside the conditional.
emacs: refactored the display of types.
emacs: C-c d now tries harder to find an atomic type.
emacs: displays the pending errors in the margin.
emacs: correctly handles warning.
emacs: cleaning and updating documentation.
emacs: toward a cleaner interface to multi-buffer.

For now there is only one merlin instance per emacs instance (instead of one per
buffer), but in the future it could be extended to have one merlin per project.
emacs: all the logic to choose a merlin instance for a given buffer is in `merlin-get-instance-name'
emacs: Documentation.
emacs: when feeding path to merlin, prepend the PWD so that it does not depend on the pwd of the merlin instance
Merge branch 'master' into experimental
Adjust sync heuristic... Need cleanup.
vim: fix python errors [<-- py sucks]
emacs: fixes old bug when feeding phrases in several steps resulted in confusing merlin.
emacs: Patch by oandrieu to allow parsing of .merlin files in parent directories.

Closes #23.
Closes #18.
Merge branch 'experimental'