Add the `System::boot()` method
Port the changes from contao/core@8151215705eb1e40ed27c5aa58ef6d978d453a24
Port the changes from contao/core@6046665ccb793a3cc204b328719dbfe63eaab644
Port the changes from contao/core@c35d1e426469fa10eb810cd70b39e22046b24c02
Port the changes from contao/core@68a98ff9c105b51cd96be7f9d7d47126f33110b1
Port the changes from contao/core@1ce210d917073da11cce936fe39d2aaf50b92fa2
Port the changes from contao/core@e2120df99f867194a4685e228bd4551cbc96d75e
Port the changes from contao/core@11b6b0f13818d86a0300f88f8390a48ceb7f1a8f
Port the changes from contao/core@441658f737aa2efc91aa7e35e68557c09c05a691
Merge branch 'develop' into feature/symfony2

Conflicts:
	composer.json
	composer.lock
	system/initialize.php