Added initial draft of converter functionality
Merge branch 'master' into convert
Added draft for Header class
Store header as Header obj
Added Set class; added xml and raw properties
New package structure

- collects utility functions in util.py
- collects class models in models.py
bumped pylint score to 8.86 :)
Add iter methods to sets and headers; add str and unicode methods
Major refactoring

- OAI methods now return object representations/iterators directly
- OAIIterator supports all verbs except Identify
- OAIIterator now uses iterfind to save memor
- Raise OAI exceptions
Defined OAI-specific exceptions