XEP-0115: Entity capabilities

[jcbrand]

As asked for by @GreenLunar (in #123):

• Please add caps (capabilities) and https://conversejs.org/ to caps in </presence>. So developers and others would know where to reach to you. See Update link in XMPPCapabilities.m robbiehanson/XMPPFramework#262. Edit: See also XMPPCapabilities.h (line 47).

http://xmpp.org/extensions/xep-0115.html

[devurandom]

This is e.g. required to support XEP-0166 (Jingle), and thus needed for #447. Several other XEPs also use this mechanism to determine what features their peers support.