Normalize and prune top-level APIs

[sethmlarson]

• Added es_ prefix to top-level APIs that aren't to do with Pandas compatibility.
• Deprecated and renamed [DataFrame, Series].info_es() to es_info().
• Renamed eland.Index.index_field to eland.Index.es_index_field
• Renamed eland.[DataFrame, Series]._to_pandas() to to_pandas()
  • Rationale: This API should be public as it's more convenient than calling eland_to_pandas(). Will need to add a circuit breaker to prevent users from unknowingly dumping their ES index to memory.
• Changed query_compiler constructors for DataFrame and Series into a private interface (_query_compiler).
• Deprecated eland.read_es() in favor of eland.DataFrame()
  • Rationale: "read" gives the idea that we're moving data around, but creating an eland.DataFrame() doesn't load any data by default.
• Deprecated eland.read_csv() in favor of eland.csv_to_eland()
  • Rationale: Mirrors eland.pandas_to_eland() where data is inserted into ES and a DataFrame is returned.
• Moved ETL functionality to its own module etl.py and left utils.py to be utilities for the module rather than users.
• Added type-checking eland/etl.py

[stevedodson]

LGTM - great cleanup!