Allow escalus_tcp to filter stanzas
Add 'not'/1 higher-order function to escalus_pred
Sanitize Escalus config global variable names

- `escalus_server` - the XMPP domain used by the system
- `escalus_host` - the machine to connect to

In case of unavailable `escalus_host` the host to connect to is
`escalus_server`. It was the other way around - quite counter-intuitive.
Add todos about the configuration
Use the domain/server to setup the stream

The host should only be used for connecting the TCP socket.
As soon as XMPP is involved the XMPP domain (also called server) is
important.
Allow the whole range of inet addresses as host
Fix whitespace
Expect Config to be passed to is_mod_register_enabled

External API change - bump version number.
Revert 65e51c84: filtering in escalus_tcp

This was not a well thought out idea.
The functionality this changeset intended to make feasible was
achieved in a simpler way from within the relevant testsuite.

OTOH if the change is desirable the code remains in the history.
Keep in mind, though, that support for filtering should be added to all
the existing transports in parallel.
Add escalus_bosh:resend_raw/2 for testing retransmissions
Change send/2 to return error if one occurs
Add XEP-0184 stanzas
Extend XEP-0184 capabilities
Fix 0184 groupchat and message types handling
Convert 0184 functions to #xmlel{}
Update .travis.yml
rebar 2.2.0
Add is_groupchat_message predicate
Merge pull request #13 from esl/newer_rebar

rebar 2.2.0
Merge pull request #14 from dcorbacho/groupchat

Add is_groupchat_message predicate