Add a "container" option instead of using $('body')

Allow usage of another container that the cached version of body (moreover that can change with some SPA)
Allow DOM selector in container option and add tests
Merge branch 'patch-1' of git://github.com/sroze/bootbox into sroze-patch-1

Conflicts:
	tests/defaults.test.js
[GH-147] tweak container tests a little