Support for Java 7

[sbaum1994]

We currently do not support Java 7, meaning the special agent also does not support Java 7.
Requesting on behalf of customers who have asked for both:

• Java 7 tracer for instrumenting
• Java 7 tracer for auto-instrumenting with java special agent

[sbaum1994]

For LightStep users who are looking for an alternative, we recommend in the mean time auto-instrumenting with datadog java which supports jre1.7. See documentation for doing that here.

This means downstream services will need to have b3 header extractors enabled for injecting and extracting of span context to work, using a b3 propagator (ex). Currently only some LightStep tracers support b3 propagation, but soon all tracers will support this propagation method.