Set namespace alias by `alias=namespace_uri`

Normally, without namespace alias will set parameter name to ```openid.ax.type.ext#{number}```.

This modification gives RP a method to set alias by:
``` option :required, ["alias=http://axschema.org/namespace/alias"] ```
OR
``` option :optional, ["alias=http://axschema.org/namespace/alias"] ```

And will set parameter name to ```openid.ax.type.alias```

```NOTE: This will let all parameters with alias act AX extension, not SREG```