Enhancement: add badges

[randy3k]

Once travis-ci, appveyor and coveralls.io are activated, these badges are ready to be displayed.

[randy3k]

As a collaborator, I actually was able to activate coveralls.io for the repo.
But the others require @divmain's owner permissions.

[divmain]

Done! The Travis build for OSX failed the first time after I merged this into master. It indicated it hadn't been able to download Sublime. That might be worth looking into if it interferes with the reliability of the CI builds. But then again, it may have been a one-time thing!

Thanks for this, looks like the badges are working really well!

[randy3k]

Thanks. I am aware that Travis OSX build does not able to download Sublime Text occasionally. The installer script actually tries 5 to improve the chance of downloading it. To my experiences, it only happens when frequent commits are submitted, but it doesn't mean that it will never happen again..

My hypothesis is that http://sublimetext.com is being unresponsive.

[randy3k]

I am quite sure it was a problem of sublimetext.com. Appveyor download was succeeded at its fifth trial.