Add [DataFrame, Series].filter()

[sethmlarson]

• Adds support for .filter() on DataFrame and Series
• Supports items, like, and regex for axis=columns on DataFrame.filter()
• Supports items for axis=index on `Series.filter()
• Renames Series.field_name to Series.es_field_name
• Fixes a bug where setting DataFrame.columns to [] would still query all dataframe columns

Closes #198

[stevedodson]

LGTM - nice new features, making progress!

[stevedodson]

Note, this assumes that _id is the current index. If the index is changed to a time series or other, we may support this.