Fixes MAHOUT-1156

Adding Searcher, UpdatableSearcher abstract classes defining what a
nearest-neighbor searcher does.

The following implementations are available in the o.a.m.math.neighborhood
package:
- BruteSearch
- ProjectionSearch
- FastProjectionSearch

Additionally there are 2 new abstract classes available:
- Searcher
- UpdatableSearcher



git-svn-id: https://svn.apache.org/repos/asf/mahout/trunk@1479307 13f79535-47bb-0310-9956-ffa450edef68