Exclude main class #68

A yet another dir structure for issues #68. Separated into document-validator-app and document-validator-core. 
document-validator-core contains only the lib jar files, and document-validator-app contains bin/, sample/, a main-class jar file, and core jar files.

The changes affect "Running examples" in the document "Quick Start", and an alteration we need is as follows:

 $ cd document-validator-app/target/
 $ tar xzvf document-validator-app-*-assembled.tar.gz
 $ cd document-validator-app-*-assembled

Request for your comment. Thanks!