Add support for .jar plugins

[Scottmitch]

Motivation:
Proto only supports plugins as single file executables, and
protobuf-gradle-plugin follows the same pattern. This makes it difficult
to use a plugin that relies on a runtime (e.g. jvm).

Modifications:

• If the plugin is a jar file, create a trampoline script as the protoc
  executable which executes the jar file

Result:
Fixes #168

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[google-cla]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

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Corporate signers

• Your company has a Point of Contact who decides which employees are authorized to participate. Ask your POC to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the Google project maintainer to go/cla#troubleshoot (Public version).
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[Scottmitch]

re-opened under #423