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.