Use Maven for build system, remove binaries from archive

Hello,

We have updated Nexmo's JAVA library to use Maven as the build system. This is a first step towards deploying the library to the global Maven repository, which will make the library more accessible to other users.

I have bumped the version to 1.5-SNAPSHOT, since 1.4 has already been release. The source code has been moved to the maven standard directory structure, and the build directory has been removed and ignored.

I would recommend in the future also removing the javadoc and compiled .jar file from the source code.