set QuakeML-valid resource identifiers on init
don't initialize a new ResourceIdentifier for event classes if
`resource_id=False` is specified
handle the forced initilization of resource_id via kwarg
documentation for previous change
Merge branch 'master' into event_set_resource_id_on_init
positional args should better be avoided in event type classes from
class factory
don't know why this was found to cause problems in 1dc4f92a3950 but on
current python 2.7 lists of lines should go into difflib (and not one
long string)
fix (doc)tests, fix resource_id handling when reading a Comment
dont force new ID on clear()
Merge pull request #661 from megies/event_set_resource_id_on_init

set QuakeML-valid resource identifiers on init