update code.goole.com/p/go.net to golang.org/x/net

https://groups.google.com/forum/#!topic/golang-nuts/eD8dh3T9yyA
Update changelog for 0.5
Merge pull request #2187 from eparis/go.net-rename

update code.goole.com/p/go.net to golang.org/x/net
Merge pull request #2434 from brendandburns/release

Update changelog for 0.5
Merge pull request #2444 from bgrant0607/docfix

Add descriptions to all v1beta1 API fields.
Document v1beta2 fields