New HDFS browser file page displaying both file content and schema

Fixes Issue #37.

 * Add a new file page to display both file contents and file schema if available. If the specific file viewer for the file does not support getting the file schema, then Schema tab is not displayed.
 * Add support for obtaining file schema for Avro and Parquet file viewers.
 * Refactor HDFS browser servlet and add JavaScript to retrieve file contents and schema via AJAX calls.
 * Change the default number of lines returned to display file content from 1000 to 100.
 * Refactor file viewers.