Implement the Military Grid Reference System for tiling [LUCENE-2646]

The current tile based system in Lucene is broken. We should standardize on a common way of labeling grids and provide that as an option. Based on previous conversations with Ryan McKinley and Chris Male, it seems the Military Grid Reference System (http://en.wikipedia.org/wiki/Military_grid_reference_system) is a good candidate for the replacement due to its standard use of metric tiles of increasing orders of magnitude (1, 10, 100, 1000, etc.)

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Migrated from [LUCENE-2646](https://issues.apache.org/jira/browse/LUCENE-2646) by Grant Ingersoll (@gsingers), 1 vote, updated Dec 02 2013
Linked issues:
- [SOLR-773](https://issues.apache.org/jira/browse/SOLR-773)

The issue names no files, tests, or entry points. Start by reviewing Lucene’s current tile-based system and the linked LUCENE-2646 and SOLR-773 discussions, then establish where grid labeling is configured. Done means MGRS is available as a standardized tiling option with its intended tile orders and behavior verified.

Written by the indexing model from the issue text.