SGF-315 - Re-activated Bundlor plugin to generate OSGi metadata.

Re-enabled the plugin and pointed it to the already existing template.mf. Polished up the template and make sure it refers to matching versions.
Re-orderd the SDG JAR Bundle Manifest.mf file OSGi-based Bundle headers.
Prepare next development iteration... the 2.0.0.M1 release.
SGF-317 - Improved GemfireCache for Spring 4.1 compatibility.

Added putIfAbsent(…) to GemfireCache to make sure we can compile against Spring 4.1. Improved the implementation of get(Object, Class<T>) to adhere to the contract defined by the interface.