Fix Documentation Formatting

I noticed that the documentation for the package was a little off when viewed in a godoc tool, such as godoc.org: http://godoc.org/github.com/mrjones/oauth 

godoc doesn't seem to provide support for HTML lists or special formatting for function parameters/return values, so to compensate for this I used indentation to place them in code blocks where they retain their formatting. This is used in a few of the core packages such as [fmt](http://godoc.org/fmt) and [ecoding/xml](http://godoc.org/encoding/xml).

You can view the result of these changes here: http://godoc.org/github.com/Forestmb/oauth