Create compiler manifest #435

The manifest should capture the information about the modules compiled and the output for that module. Ideally, it would have the following information.

• Module name
• Beam file path
• Beam file timestamp
• JS file path
• JS file timestamp

It should also capture the compiler version.

Mix.Project.manifest_path will show the path to where manifest files are expected to be. Elixir saves it's manifest files as .compile.elixir so ElixirScript should save its own as .compiler.elixir_script