Implement FrameManager

This patch implements FrameManager which is responsible for maintaining
the frame tree. FrameManager is quite basic: it sends FrameAttached,
FrameDetached and FrameNavigated events, and can report mainFrame and
all frames.

The next step would be moving certain Page API's to the Frame. For
example, such method as Page.evaluate, Page.navigate and others should
be available on Frame object as well.

References #4

test/assets/frames/script.js

@@ -0,0 +1 @@
+console.log('Cheers!');