HM/TJ: field without term-frequency is possible.
HM/TJ: move createField and friends to a factory object.

The new FieldFactory will give room to add specific types for custom fieldnames. Field type selection can be changed to something other than name prefix.
HM/TJ: refactored FieldFactory, removed TIMESTAMPFIELD.
HM/TJ: NoTermFrequencyField can be created by the FieldFactory.
HM/TJ: FieldFactory also register tokenization and phrase-query-possibility.
TJ: freeze factory, use isUntokenized from FieldFactory.'