Added sample 'star schema' dwca file.
Initial support for DwCA extensions.
Example file now demonstrate access to extension data.
API additions: added from_source() and from_extension() to DwCALine.
API additions: added from_source() and from_extension() to DwCALine.
Added new API example in example.py
API improvement: turned from_core() and from_extension() from method to properties.
Tiny PEP8 fix.
API improvement: line content is accessible through the 'data' dict rather than via a get() method.