Report setup_all errors and mark tests as invalid

Closes #878
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.
Use in_project in deps.compile (tks @ericmj)
Remove outdate docs and example
Add String.to_integer/1 and String.to_float/1

Given a string that starts with a valid integer, String.to_integer/1
returns a tuple {result, remainder} where remainder is the character
string following the valid integer. If the input is not valid as
specified by Erlang's :string.to_integer/1, the function returns
:error.

Given a string that starts with a valid float, String.to_float/1
returns a tuple {result, remainder} where remainder is the character
string following the valid float. If the input is not valid as
specified by Erlang's :string.to_float/1, the function then calls
String.to_integer/1 and returns that value, which is either an
{integer result, remainder} or :error.

This latter behavior allows you to say String.to_float("34")
without generating an error.
Make String.to_float always return a float

Added code to make String.to_float("34") return {34.0, ""}
Add tests for String.to_integer and String.to_float
Remove unnecessary task handling deps.compile
Add --umbrella option to 'mix new'
Add '--quiet' option to 'mix deps.get'

Closes #1028.
Merge pull request #1049 from ericmj/deps-get-quiet

Add '--quiet' option to 'mix deps.get'
Improve function(some_macro/x) error message
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