Remove unnecessary task handling deps.compile
Add --umbrella option to 'mix new'
Remove :app project setting in umbrella projects
Improve cons cell and other operators precedence, closes #862
Use a manifest to store generated files

This allows us to remove trailing .beam files
when a file is removed and more robust compilation
detection.

Closes #717, closes #954
Merge pull request #1048 from ericmj/mix-new-umbrella

Add --umbrella option to 'mix new'
Improve warnings related to default arguments

* "default arguments in some_fun/3 are never used"
* "the first default argument in some_fun/3 is never used"
* "the first 2 default arguments in some_fun/3 are never used"

Closes #929
Clean up and speed up exceptions