Make caller serialization configurable

[skipor]

$GOPATH trim logic based on go-stack/stack which is under Apache license, so can't be put in repo. So required logic have been moved to another repo (skipor/goenv), and exported as third party.
That decision is doubtful, so fell free to propose something better.
WARN: skipor/goenv is not pinned in glide.lock. I don't know how to do this without updating and keeping glide.lock hash valid.

[mention-bot]

@skipor, thanks for your PR! By analyzing the history of the files in this pull request, we identified @akshayjshah, @peter-edge and @imkira to be potential reviewers.

[bufdev]

I don't think copying someone else's code out to your own repo is a nice thing to do, also you just copied the license so this doesn't change anything. We were thinking of using Chris Hines' code in #311. This code somewhat duplicates that effort as well.

[skipor]

Well, I remove GoPathCallerEncoder commit with external dependency.
This PR still fixes #319, and nice start point for #304.

[flyingmutant]

Does this PR have a chance to land before 1.0?

[skipor]

@peter-edge, @akshayjshah need I fix something, or your will handle this PR yourself?

[akshayjshah]

There's certainly a chance that this can land before 1.0; it's not a breaking change, so we can also land it after 1.0 if necessary.

@skipor we can take it from here - I just need to spend the time to review it carefully :)

[akshayjshah]

LGTM, just going to push some small cleanups.