Revert JCLOUDS-652 so that we don't require java 7

This reverts the change that required users to use java language 7.
https://issues.apache.org/jira/browse/JCLOUDS-652

This takes care of the default http driver and single-shot 2GB fixed length posts. Basically, those using JRE7 will work. Those not, can choose a different http driver.
https://issues.apache.org/jira/browse/JCLOUDS-264

Note: this also permits special casing of modules that need Java 7. For example, the filesystem api is configured so that if the JRE is at least 7, it will build.

Actually, comment about this: could we not check here whether we are running on a 6 or 7 JDK? If we're on a 6 JDK, we _know_ the reflective call cannot succeed, so why try?

Or am I missing something here..?