FastVectorHighlighter: Make FieldFragList "plugable"

This patch contains:

• Introduction of abstract class BaseFragListBuilder

• SimpleFragListBuilder now extends BaseFragListBuilder

• FieldFragList is now abstract

• Introduction of SimpleFieldFragList, a simple implementation of FieldFragList

• Some necessary, minor changes

This is the second (see LUCENE-4091) patch aiming at the introduction of another method for weighting fragments (see LUCENE-3440).

Tests were okay.