Added explicit null return to getConfig if the config item requested doesn't exist
Updated PhpDocs for setConfig() and error()
Updated PhpDocs for __construct() and notImplemented()
Updated PhpDocs for Plugin::__construct(), View::__construct(), View::get() and View::render()
Added output buffering to App::serve() to allow additional headers to be set at any point in the page generation. This fixes a bug with nesting one View inside another, caused by the View attempting to set a header when it renders
Minor code cleanups
Added Swiftlet\Exception, tests