web_router is a routing server that uses mnesia to keep track of
exchanges, routes and callbacks.

Callbacks are funs.  Applicatons/Processes/Other tell web_router
to invoke a callback on a certain routing key for a certain exchange.

If your exchange was "example.com" and you had an about page and a
nyse stock page you might have callbacks assigned to the routes:

<<"about">> and <<"stock.nyse">>.

Routing keys should in general be binaries (use web_router:key as
a helper method) and can be anything.  Typical usage would be
to take each '/' in a url and turn it into a '.'

'users/12' becomes 'users.12'

For Restful interface you may want a callback to serve any user id
so you invoke a callback on the routing key 'users.*'.

* matches any one section.
# matches one or more section.

'users.*' matches 'users.12' but not 'users.12.edit'
'users.#' matches 'users.12' and 'users.12.edit'
'users.*.edit' would match 'users.12.edit'


Adding Exchanges:

1> web_router_exchange:declare(ExchangeName, Type, Durable, AutoDelete)
#exchange

ExchangeName is the name of your exchange (any term)

Type can be topic, direct or fanout.  Currently only topic is supported.

Durable tells web_router to keep the exchange stored on disk so if
the server crashes it can be automaticaly recovered when it comes back
online.

AutoDelete would delete an exchange if no callbacks existed for it but
is currently not implemented.


Adding Callbacks (bindings):

2> web_router_exchange:add_binding(ExchangeName, Callback, RoutingKey)

Callback is a fun.
RoutingKey is a binary key to match against (<<"users.12">> or
<<"get.forums.12.topics">> etc)


Calling a route:

3> web_router_exchange:route(#exchange, RoutingKey, Args)

#exchange is the exchange record returned by declare.  Later
this may allow you to simply specify a name instead of a record.

Args is a list of arguments to pass to the callback.

Currently if multiple callbacks are assigned to the key it returns an
array of what the callbacks return.  This may be subject to change.