Added filters and message formats
Fixed error in calling _filter_message()
_check_format() fixed
Message format check converted to filter

Now filters may raise FilterError to signal a processing error
Added comment
Documentation updated
Removed incomplete format checking code
Merge branch 'feature/filters' into develop
Version bumped
Merge branch 'release/1.1.0'
Merge tag '1.1.0' into develop

Added filters