fix issue #772
update contributing.md
Replace all deprecated sendRequest() calls with new sendMessage()
Add onMessage() stub to Chrome extension API mock
Content script should handle message even when there is no tab property
Merge pull request #829 from sainaen/fix_820

Replace all deprecated sendRequest() calls with new sendMessage() ones
Merge pull request #825 from UncleBill/update-contributing

update contributing.md
Merge pull request #816 from UncleBill/master

Fix issue #772
Whitespace
Tweak to make if statement positively formed