Add tracer.NewChildSpanFromContext

[elijahandrews]

This PR addresses #65

I've created tracer.Span, which is allows us to replace:

span := tracer.NewChildSpanFromContext("name", ctx)
ctx = span.Context(ctx)

with:

span, ctx := tracer.NewChildSpanWithContext("name", ctx)

[elijahandrews]

Hm, from the build:

tracer/example_context_test.go:18: too many arguments to conversion to tracer.Span: tracer.Span("filestore.saveFile", ctx)

Looks like the function is colliding with the Span type. What should we do about that? Do we need to hack something with DefaultTracer to get this working?

[clutchski]

+1