Adding applications to .app.src for systools compatibility.

This patch allows relx to properly generate releases with cassandra_thrift.

As of 13 Nov, 2013 the required for systools to work properly are:

 * description
 * vsn
 * modules
 * registered
 * applications

See http://www.erlang.org/doc/man/app.html
Merge pull request #1 from potatosalad/systools-compat

Adding applications to .app.src for systools compatibility.