Refactor building of elasticsearch body

[amymok]

Pulled out the elasticsearch body building to separate body builders to make it easier to identify the different building parts.

This PR branches off from PR #13, you can start reviewing from commit e9af19b until that PR is merged.

Additions:

• Added builders to separate out building search, post-filter, and aggregation body

Removals:

• Remove building search, post-filter, aggregation body in es_interface.py

[coveralls]

Coverage remained the same at 100.0% when pulling 58977de on amymok:refactor-esbody into f2a3503 on cfpb:master.

[JeffreyMFarley]

This is a better organization