handle custom CouchdDB welcome page

This change modify behaviour of Welcome handler by allowing admin to customize the welcome page. If content is "application/json" it will send current json object, if not, it will send an html page or redirect to another path.

This patch change the confugration string to a tupple

{WelcomeMsg, WelcomePage}

. WelcomePage could be a string for DocumentRoot where it will find welcome.html page or a tupple

{redirect, Url}

Ex if in config you have:
/ = {couch_httpd_misc_handlers, handle_welcome_req, {<<"Welcome">>, "/usr/local/share/couchdb/www"}}

If Accept content is application/json Couchdb will send :
"couchdb":"Welcome","version":"0.10.0a"}

If other it will send welcome.html.

if you replace "/usr/local/share/couchdb/www" by

{redirect, <<"/_utils">>}

 user will be redirected on futon.