fixup [filter_vision] tags

This is the master version of PR #237, with unit tests.
add [has_ally],[has_enemy] tags for standard side filter

Unlike [allied_with] and [enemy_of], these will match if only if
there is *any* ally or enemy matching the filter. The other versions
of the tag match only if all sides matching the filter are
allied / opposed.

This commit also adds a unit test that checks that it works as
expected.
update changelog
Merge branch 'has_ally_master'

Conflicts:
	wml_test_schedule
update changelog
Merge branch 'master' of git://github.com/wesnoth/wesnoth

Conflicts:
	changelog