Move mixins
doc cleanup
Updated documentation links

Links to the elasticsearch.js docs are out of date.
Updated another link
Partial fields support (source filtering)
Source filtering
Terms facets support for shard_size parameter.
Initial aggregations support.

Added Global, Filter, and Terms aggregations.
Add aggregation tests to Request.
Add GeoHashGridAggregation
Added HistogramAggregation
Added MissingAggregation
Added NestedAggregation
Added RangeAggregation
Added SignificantTermsAggregation
Add MetricsAggregationMixin and AvgAggregation.

The Metrics Aggregation Mixin can be used as the base
for most metrics based aggregations.  The AvgAggregation
uses the metrics aggregation mixin for all methods.
Added CardinalityAggregation
Added MaxAggregation
Added MinAggregation
Added PercentilesAggregation