Vendor dependencies as git submodules

[mistydemeo]

This is one of two branches I'm submitting as proposals for vendoring Siegfried's dependencies in the Go 1.5+-supported way, which will simplify the process of building packages.

This PR vendors dependencies by using git submodules. #71 vendors the entire source trees of all of the dependencies instead.

Advantages:

• There's minimal overhead in terms of repository size.

Disadvantages:

• The checked-out repo can't actually be built until the submodules are fetched and updated.
• The Github-generated tarball won't contain the dependencies, since it doesn't contain submodules.

[richardlehane]

Going with full vendoring... Thanks!