[SPARK-2220][SQL] Fixes remaining Hive commands

This PR adds support for the `ADD FILE` Hive command, and removes `ShellCommand` and `SourceCommand`. The reason is described in [this SPARK-2220 comment](https://issues.apache.org/jira/browse/SPARK-2220?focusedCommentId=14191841&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-14191841).