Validate pre-2.0 coprocessors against HBase 2.0+

We have co-processors for a while, but the API has been changed recently. We should give some tooling for our users to determine if they can use the previous co-processors safely or not.

The tool should:

• try to load the co-processors on our current classpath for ensuring class references are on our classpath,
• should check for previously removed co-processor methods.

In this version we check only method signatures. Code references should be checked in further versions.