Bucketing of association value of a certain category [LUCENE-3688]

Add facet requests which collect associations of a certain category into buckets, and returns each bucket as a facet result node. Association type is either int or float, and there are two methods to define buckets. The first by providing buckets which contain pre-defined ranges. The other is by declaring the required number of buckets, where the ranges of the different buckets are dynamicly set to create balanced bucket sizes.

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Migrated from [LUCENE-3688](https://issues.apache.org/jira/browse/LUCENE-3688) by Sivan Yogev, updated Jul 22 2013
Attachments: [LUCENE-association-buckets.patch](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-3688/LUCENE-association-buckets.patch), [LUCENE-association-buckets.r1292224.patch](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-3688/LUCENE-association-buckets.r1292224.patch)

Start by reviewing the linked LUCENE-3688 issue and the attached LUCENE-association-buckets.patch files, then locate the existing association facet request and result-node APIs. Done means supporting int and float associations with both predefined ranges and a requested number of dynamically balanced buckets, returning each bucket as a facet result node.

Written by the indexing model from the issue text.