Don't warn on unused imports
Automatically detect when to warn on import
Add support for umbrella project as dependency

Closes #1031
Fix mix function names and docs
Fix problem with an in-dep rebar not being found—need to pass in the full path to it, and not just ./rebar
Merge pull request #1042 from pragdave/master

One liner in mix do_rebar in deps.compile
Merge pull request #1040 from ericmj/umbrella-as-dep

Add support for umbrella project as dependency
Merge pull request #1026 from elixir-lang/jv-no-warn-unused-imports

Warn on unused imports
Add documentation about import warnings
Make Mix.Project.load_project private in favor of in_project
Move the umbrella check to Mix.Project.recur

We have named the function from Mix.Project.recursive
to recur in order to imply action.