Use cleaner Messenger + ignore non-handled messages

[fregante]

Context:

I reorganized the messenger and it seems to work correctly for the existing use cases. I'd like to merge this soon so we can get some testing done.

Based on:

• Reorganize extension for clarity webext-messenger#49

Changes other than reorganizing of the code:

• Completely ignore non-handled messaged
• Slightly improve the output of logging

Here you can see some items from the content script:

• the CS sends MARK_TAB_AS_READY to "runtime" and receives its response (neither one was logged before)
• the CS receives DETECT_FRAMEWORKS, handles it, and responds

Next:

• Provide way to correlate messages/responses in logs webext-messenger#50
• Provide way to see unified log of message events webext-messenger#48

[twschiller]

[fregante]

You may merge at your earliest convenience. I just published the same exact version as "stable"