Do not load mix.exs for mix deps coming from hex

This is an optimization that allows us to shave off some
seconds from booting applications.
Ensure defoverridable is not order dependant, closes #2615
Accept upcoming hex formats
Allow `elixirc_paths` to also be given through the command line

Once given, the command line overrides the project
configuration in `mix compile.elixir`.

Closes #2557
Validate app and module on mix new
Merge pull request #2618 from patrickgombert/mix-new

Validate app and module on mix new
Improve mix new error messages
Avoid race conditions in the struct detection
Print app name before compiling Makefile deps

This should close out #2616.
Merge pull request #2619 from knewter/feature/2616_print_app_name_before_compiling_makefile_dependencies

Print app name before compiling Makefile deps
Print app name on Mix.shell.cmd/1/2
Add a test case for printing samples on IO shell
Bump hex requirement
Push the given app name as default on Mix.Project
Use Mix.Shell.printable_app_name
Print a nice prelude if we fail to load a project, closes #2430
Release v0.15.0
Start v0.15.1-dev
Remove deprecated code
Do not normalize paths on join, closes #2500