Adding a Installation_Manager to facilitate installation

[srtfisher]

Adds an installation manager to allow folks to fluently install the Mantle Testing Framework and define before/after hooks.

Previously

Mantle\Testing\install(
	function() {
		tests_add_filter( 'muplugins_loaded', '_manually_load_environment' );
	}
);

New

Mantle\Testing\manager()
	->before( ... )
	->after( ... )
	->loaded( ... )
	->install();

I've also combined the logic of mantle_after_wordpress_install to this manager (it's a singleton) but the logic of mantle_after_wordpress_install will still continue to exist.

[anubisthejackle]

Looks good. I like the fluent API. Added a suggestion for passing $accepted_args but that's all I can see.