#9 add filterDefinition & resolve filter details

[JoernBerkefeld]

PR details

What is the purpose of this pull request? (put an "X" next to an item)

• [x] New metadata support
• [x] Enhanced metadata

What changes did you make? (Give an overview)

• close #9
• added filterDefinition
• upgraded filter to actually resolve details during retrieve/deploy ('filterDefinition', 'list', 'dataExtension', 'folder')
• adjusted what fields are actually creatable / updatable / retrievable

To-Do

• [x] retrieve filter & resolve all details
• [x] retrieve filterDefinition & resolve all details
• [x] resolve filter in automation
• [ ] create filter
• [ ] create filterDefinition
• [ ] update filter
• [ ] update filterDefinition
• [ ] delete filter
• [ ] delete filterDefiniton

Checklist

• [ ] I have performed a self-review of my own code
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] test scripts updated
• [ ] Wiki updated (if applicable)