Add JAX-RS Callback

[krisgerhard]

Goal

JAX-RS is a very popular API which is yet to be out-of-the-box supported by Bugsnag.

Design

The design very highly mimics the Java Servlet API Callback implementation.

Minimal Java version: 8

Testing

I've been using this in production for about two weeks as of today.

[krisgerhard]

Somewhy Travis build was not triggered

[krisgerhard]

Well... okay. If this repository wants to be java 7 compatible I have nothing to offer :(

[abigailbramble]

Hi @krisgerhard thanks for raising this and submitting this PR. We will take a look at this as priorities allow.