Move dependencies callbacks back into Mix.Deps.Converger

The converger is the one that knows if a dependency exists or
not and, as such, is the best one to know if a dependency callback
should be invoked or not.

This commit also normalizes some of the Retriever internal
functions, for example, now we have mix_children and rebar_children.
The goal is to extract those functions (and the ones in Mix.Deps)
one by one into a Mix.DepsManager behaviour which will be implemented
by Mix.DepsManager.Mix and Mix.DepsManager.Rebar, removing all
the conditional calls existing in the code.
Correctly add load_paths for rebar dependencies

Signed-off-by: José Valim <jose.valim@plataformatec.com.br>
Do not access another task directly, use Mix.Rebar module instead
Don't ignore nil during aliasing. Attempt to fix #1159
Merge pull request #1160 from jjh42/alias-nil-correctly

Don't ignore nil during aliasing. Attempt to fix #1159
By default, start the application on escript
Improve docs for IEx helper l/1
capture_io captures a prompt when capturing of stdio
Merge pull request #1167 from mururu/prompt

capture_io captures a prompt when capturing of stdio
Fix typo on |> docs, closes #1168
Rebar dep with no config is given rebar as manager
Speed up escriptize tests
Set load paths for dependencies with Makefile

A recent update (036d016 2013-06-02; Rebar dep with no config is given
rebar as manager) introduced :make as a value for a dependency's
manager, but Mix.Deps.load_path has no function clause to handle this.

Add :make to the nil manager clause, as it seems the closest
equivalent.
Fix IEx's module source lookup

Module.module_info(:compile)[:source] returns the value, not a tuple
as the code was expecting, making r/1 think that no module has a
source. Fix this expectation.
Merge pull request #1172 from carlosmn/iex-r

Fix IEx's module source lookup
Merge pull request #1170 from carlosmn/load-path-make

Set load paths for dependencies with Makefile
Option that enable to decide whether `capture_io` captures a prompt or not
Merge pull request #1174 from mururu/capture_prompt_option

Option that enable to decide whether `capture_io` captures a prompt or not
Ensure noname scope does not leak after assignment
Implementation of a basic test framework for IEx. Closes #919