mod_http_oauth2: Add a special "xmpp" scope that grants the users' default role This will be the first step towards defining a standard set of XMPP scopes. "xmpp" behaves as an alias for the user's default role, so that the client does not need to know about the various prosody:* roles.

This module logs events where more than a certain amount of CPU time was
spent.

``` lua
log_cpu_threshold = 0.01 -- in seconds, so this is 10 milliseconds
```

Uses the Lua
[`os.clock()`](http://www.lua.org/manual/5.2/manual.html#pdf-os.clock)
function to estimate CPU usage.