Adding improved error message when multiple assembly jars are present.

This can happen easily if building different hadoop versions.
A little revise for the document
Merge pull request #114 from soulmachine/master

A little revise for the document
Merge pull request #113 from pwendell/master

Improve error message when multiple assembly jars are present.

This can happen easily if building different hadoop versions. Right now it gives a class not found exception.