Fix #259: HTML response to EWS autodiscover triggers libxml2 assert

Add "Accept: text/xml" header to EWS POST request to reject all other
content types in response.
Fix #259: HTML response to EWS autodiscover triggers libxml2 assert (II)

It seems that some HTTP servers ignore the "Accept:" header. Added a
check that the "Content-Type:" header starts with "text/xml" before we
call the libxml2 parser.

NOTE: I couldn't reproduce the libxml2 assert on Fedora 20 from the
provided debug log. It simply complains about XML errors and returns
a parse error to the caller.
Fix #258: V&V call gets rejected when IPv6 is enabled

commit 1556f377ecc7c3d4887f8e9f2805a4e7f7e60660 changed the candidates
list to be sorted when it is created and *NOT* sort it when it is
converted to the SDP message. Unfortunately that changed the value of
msg->ip in sipe_media_to_sdpmsg() [through backend_stream_to_sdpmedia()
and backend_candidates_to_sdpcandidate()] in the case when an IPv6
address was assigned to the host.

As the media code does not support IPv6 addresses yet, we now reject IP
address strings with ":" in get_stream_ip_and_ports().
OBS: sync with latest versions
utils: fix typos
media: ignore IPv6 candidates
srtp: parse encryption key from SDP
srtp: generate encryption key for each stream
srtp: offer encryption key when initiating a call
srtp: implement sipe_backend_media_set_encryption_keys()
notify: store ucPC2PCAVEncryption value form server
srtp: add sipe_backend_media_get_encryption_policy()
media: send encryption SDP attribute based on policy
media: add stream_decryption_keys list to sipe_media_private
media: refactor encryption compatibility check
srtp: negotiate encryption and apply keys to backend
srtp: encryption policies WIP
media: mux RTP and RTCP when using TCP connection

Lync uses a single TCP connection for both components.
media: duplicate TCP candidates
filetransfer: use 'init' callback in sipe_file_transfer

Replaces sipe_core_ft_incoming_init() and sipe_core_ft_outgoing_init()
and the specific callback method is now set by Sipe core depending on
whether incoming or outgoing file transfer is being handled.

Will allow for some code to be shared between legacy OCS file transfer
implementation and the new Lync approach.