Added hexchat_hook_server_attrs() and hexchat_hook_print_attrs() to the plugin
interface. This hooks are similar to hexchat_hook_{server,print}() except the
callback passes an extra argument with the (new) structure hexchat_event_attrs.

This structure contains attributes related to the event; by now it only contains
the server_time_utc member which is non-zero if server-time is enabled and the
server used this extension to pass a timestamp.

See issue #661.

(Note: this hooks are still not called by hexchat in this commit.)
Removed two dummy functions in plugin interface.
Now hexchat_hook_server_attrs() and hexchat_hook_print_attrs() is called
when it should. This should close #661.
Indentation fixes and code cleanup.
Added hexchat_emit_print_attrs() to plugin interface.
Added functions to create/destroy event_attrs to plugin interface.

Function names were chosen to keep consistency with the rest of the
API.
Removed outdated comment.
g_free() should be used for g_malloc().
Forgot to add hexchat_emit_print_attrs() to the macros in hexchat-plugins.h.
Merge pull request #675 from orium/server-time-plugins

Server time plugins