Roster item 'ask' property is always 'subscribe' for subscription requests

* 'pending' state is denoted by subscription='none', ask='subscribe'
* 'requesting' state is denoted by subscription='from', ask='subscribe'

Reference: http://xmpp.org/rfcs/rfc3921.html#int

I'm not yet 100% sure with regards to the 'requesting' state, I can't find
anything about it in the above mentioned reference, but apparantly that's what
prosody's roster response looks like (e.g. <item jid='XYZ' ask='subscribe'
subscription='from'/> ).

With ejabberd I haven't yet received such a roster response and contact
requests are always handled via presence stanzas.

updates #81