adding modules support (nytimes#152)

* first attempt at go modules

* adding example deps

making test publisher threadsafe

Revert "Improve Prometheus metrics" (nytimes#148)

* Revert "default handler for 404 metrics to '__404__'"

This reverts commit 298b6ae.

* Revert "update comments"

This reverts commit bec4a80.

* Revert "address comments"

This reverts commit 91ef576.

* Revert "Add panic if metric was not registered."

This reverts commit 1f73b81.

* Revert "Improve Prometheus metrics"

This reverts commit 99001b6.

Added ErrMultiRegister on multiple Register calls

enhancement - update README, more condensed/readable

Allow to specify an AWS Role to be used when creating the credentials…

… with MFA support