Use extension point for gathering step implementations

Hi,

I'm working on an Eclipse RCP app that is about to gain support for gherkin files and running them in a specialized environment. Since we won't be using Cucumber-JVM the first thing that caught my eye was the hard dependency onto the jdt-steps plugin which we'd not want to ship.

Since the extension point already existed I simply added the necessary glue code to use that extension point and instantiate the implementations.  I expect that any project will not have more than one language for the step-implementations, so just asking them all for their steps and then matching that should be fine and not cause any false-positives.

Hopefully formatting is correct, unfortunately I didn't find any formatting-settings file that I could just import to get the formatting rules you use. Hence I've used the "Java" builtin formatting from Eclipse which seems to fit the code surrounding my changes.