Rename filter to filter_query

[esmarkowski]

This pull request renames the filter method to filter_query in order to resolve a conflict with the aggregation filter and query filter. The necessary changes have been made in the codebase to reflect this renaming. This will improve clarity and avoid confusion in the codebase. Fixes #45

In addition it adds AggregationMethods that add helpers for named aggregation types.

This allows easier composition of aggregations and allows nested aggregations to provided in a comprehensive manner:

Model.terms(:names, field: :name)

# with nested aggs
Model.terms(:names, {field: :name}, aggs: {avg_salary: { avg: {field: :salary}}})

Each aggregation method passes it's values to the original .aggregation with the name of the aggregation type added as a key, allowing for more concise interface. Fixes #46