Allow to specify a subfolder where to put the launched application in jnlp-war plugin

[mauromol]

See: #45 (comment)

I would like to specify a subfolder in the WAR where the generated jnlp file and lib folder are put.

That is, I would like to be able to get the following structure (let 'subfolder' be this subfolder name and ria the name of the RIA project):

META-INF/
META-INF/MANIFEST.MF
WEB-INF/
WEB-INF/lib/
WEB-INF/lib/jnlp-servlet-1.2.2.jar
WEB-INF/lib/javax.servlet-api-3.1.0.jar
WEB-INF/lib/javax.websocket-api-1.0.jar
subfolder/launch.jnlp
subfolder/lib/
subfolder/lib/ria__V1.1.0.jar.pack.gz
subfolder/lib/ria__V1.0.0__V1.1.0.diff.jar.pack.gz