Add stackTrace getter

[AmaranthineCodices]

Closes #81. Adds a stackTrace method to Component that returns the current stack trace for the instance. This will return nil when elementTracing is false.

[coveralls]

Coverage increased (+0.2%) to 90.409% when pulling 0528d1a on AmaranthineCodices:expose-stack-trace into b25a063 on Roblox:master.

[LPGhatguy]

Is there a more descriptive name we can give this? It's the stack trace of where the component's corresponding element was created.

One thing that's fascinating is that this stack trace can change over the lifetime of the component!

[AmaranthineCodices]

Yeah, naming is definitely something that can be changed...maybe something like rootElementTraceback or elementTraceback? I'm not super sure about the exact name, but it's important, since there's two or three possible stack traces a component can have >.<

[LPGhatguy]

I suspect it should have a verb, like getElementTraceback would be pretty good!

[AmaranthineCodices]

Done!

[LPGhatguy]

I keep forgetting this, but can you add an entry to CHANGELOG in the style of the other entries?

[SlartibartfastFjords]

Looks good to me!