Make tuple field naming configurable

fix #177
EsInputFormat falls back to MapWritable if needed

EsInputFormat returns by default LinkedMapWritable which preserves the
document result structure. While the class is compatible with
MapWritable due to the way Hadoop does static checks (direct class
comparison) the job fails.
Where possible, EsInputFormat should fall back to the default
MapWritable class if LinkedMapWritable is not specified.

fix #204
Minor polishing
Change version from 1.3 to 2.0 + doc updates