add missing couch_rel src.
add couch_randomdoc to the build.

Add the couch_randomdoc application to retrieve randomly a doc from a
database or a view.
fix build. couch_httpd.hrl doesn't exist anymore

For now the #httpd{} record is still in the couch application waiting
for RCOUCH-15 so use a special branch to make sure we can build this
release.