Fix documentation comments

The documentation (mostly for constructors) is broken at [dartdocs](http://www.dartdocs.org/documentation/google_oauth2_client/0.4.0/index.html#google_oauth2_client/google_oauth2_browser.GoogleOAuth2)

This PR converts some of the comments to Markdown, so that they appear ok. I added a few periods, changed a few one-liners to the third-person present tense ("take a request" --> "takes a request") and not much else.

I also added a bit to the README, where I got confused, not understanding that I needed to provide my own `oauthReady` :tongue: . Let me know if anything is not to your liking.