Allow prepending a custom user agent

Give your users the ability to prepend a custom user agent. So if the existing user agent is "jclouds/1.8.1 java/1.7.0_51"

Provide a method for your users to easily prepend their own user agent so they can further customize it. e.g. "users-app/0.1.2 jclouds/1.8.1 java/1.7.0_51"

This will give cloud providers much better insight into how people are using jclouds. This should be well documented and part of examples that are written for jclouds.

It could be set like,

ComputeServiceContext context = ContextBuilder.newBuilder(PROVIDER)
   .credentials(username, apiKey)
   .userAgent("users-app/0.1.2")
   .buildView(ComputeServiceContext.class);