Add marker metadata feature

[tenderlove]

We'd like to be able to log metadata along with profiles, for example "a request to X controller and Y action is starting now". I think this can be accomplished via "markers" that are available in Gecko profiles.

This is related to #203 and #194

[dalehamel]

along with the gecko marker support, it would be nice to take advantage of gecko's concept of "threads" to model ruby thread or fiber IDs / names. I had implemented such support in #203, which this issue now supersedes. I was reading this value each time a sample was captured though, and I don't think it quite makes sense to model this as a "marker" per se, but that might depend on the implementation you have in mind.