added loader variable typehint for easier manipulation in IDEs
merged branch thunderer/autoload-class-typehint (PR #519)

This PR was submitted for the master branch but it was merged into the 2.1 branch instead (closes #519).

Discussion
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Added $loader variable typehint for easier manipulation in IDEs.

Not all project dependencies are managed by Composer. Common solutions for (company / private) libraries and bundles are git submodules or even symlinked directories.

This PR adds typehint for `$loader` variable inside `app/autoload.php` so people won't need to search for information on what class it is and what interface it exposes when trying to add their code to autoloader.

Commits
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67e9d69 added loader variable typehint for easier manipulation in IDEs