Add ability to map field names between model and database

See tests for examples
Add magic method __unset as well as interface __unsetter

- add __unset
- add __unsetter
- deprecate _unset
Add all dataypes
Merge pull request #27 from jrschumacher/feat-unsetter

Add magic method __unset as well as interface __unsetter
Merge pull request #28 from jrschumacher/enha-references

Add all dataypes
Merge pull request #29 from jrschumacher/feat-db-field-name

Add ability to map field names between model and database