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