Add HTTP Resolver and Support alternate valid GNUMake filenames

[zph]

(Builds off #35 but will rebase it when those are in master. So only later commits are relevant)

1. Adds an http resolver to allow for:

include https://example.com/Makefile

I updated the readme to document this and it keeps backwards compatibility when include statement doesn't have a remote that starts with http.

2. Allows for alternate filenames in local repo per guidelines from GNUMake, in precedence based ordering. These are:

var makefileVariants = [3]string{"GNUmakefile", "makefile", "Makefile"}

[zph]

The tests did not mention using a special version of tree from campoy/tools/tree, until I found it in Semaphore. That explains why my test output was succeeding but then failing after getting local to match CI (by installing campoy/tools/tree).

The tests now match the tree that is available via brew install on OSX, which is the most common implementation: http://mama.indstate.edu/users/ice/tree/.

I think it makes sense to use that one. But if you'd prefer to use the campoy version, I'll undo these changes and add it to an automatic setup step :). Either way, I should include in this PR a mechanism for getting local system to easily run tests w/ any additional tooling required.

[tj]

ahhh yea I think it was missing in Semaphore, or maybe I was missing it, I forget now haha. I'm cool with whatever

[tj]

LGTM!