add rebar to the build
add couch_collate

couch_collate is a nif replacing couch_drv and couch_ejson_compare
binaries to do binary collation using ICU.
inital move to rebar compilation

- move src/apps
- download dependencies using rebar
- replace ejson by jiffy
- replace couch_drv & couch_ejson_compare by couch_collate
working build of couchjs

couchjs is now built correctly. It can be build against the libraries
already installed on the system or statically.

By default it set the CFLAGS to /usr/include/js and the LIBS to -lmozjs185.

These values can be changed by setting the environnment variables
JS_CFLAGS and JS_LIBS.
make couch_index a standalone erlang app

With this change couch_index will be started by the release instead of
letting couchdb to start it. It also has its own supervision now.
make the couch_replicator a full Erlang application
couch_drv has been replaced by couch_collate
fix couch_dbupdates warning