Added ElasticSearch Reader
Create add.extension(extenstion, reader) function

This allows the addition of custom readers.
Removed ElasticSearch reader from dependencies

Including the ES reader causes problems with travis CI

Plan is to allow additon of custom readers through packages
Load reader files in lib directory

Custom reader files can be added to the lib director
load.project will load these readers and apply them to data in the data folder
Removed elasticsearch reader

ElasitcSearch Reader is moved to https://github.com/KentonWhite/esReade

Install package and add esReader to libraries in global.dcf
Moved add.extension to .add.extension

Preface add.extension with a . to indicate that it should be treated as hidden

Exported .add.extension to indicate that this is a stable, public interface