qml: refactor default namespaces
qml: trying a simple way to enable custom additional tags (working
commit)
qml: correcting skipping present namespaces
qml: changing structure of additional info

pick.additionals: AttribtDict with field names as keys and dicts
containing "value" and "namespace"
qml: allow custom namespaces with and without given abbreviation. use

default obspy namespace when no namespace is given. currently looks up
fields in an AttribDict called obj.extra. Currently for pick and event
only.
qml: Added support for attributes in extra
Update filter.py

clarifying docs
Update filter.py
modified filter docs
Merge remote-tracking branch 'upstream/master' into quakeml_namespaces

Conflicts:
	obspy/core/quakeml.py
fix writing boolean value to xml (all lowercase)
fix namespace abbreviation collisions
use an actual number in obspy default namespace
add test for writing/reading quakeml with extra tags
write extra tags of all event elements to quakeml
read extra tags in quakeml files
quakeml extra tags: automatic type conversion

- writing extra tags to quakeml: save type information as attribute
  'pythonType'
- reading extra tags from quakeml: try to restore correct python type if
  attribute is present
- +tests
fixes for py2.6 (importlib) and py3 (iteritems)
Merge remote-tracking branch 'upstream/master' into quakeml_namespaces
Merge remote-tracking branch 'upstream/master' into quakeml_namespaces