Doc improvements: Types of closure arguments

I had hard time using HTTPBUilder outside of the examples because the documentation isn't clear on what are the types of the arguments passed to various closures. It would be great to add this.

• On http://groovy.codehaus.org/modules/http-builder/doc/index.html it's mentioned that 'req' is of type 'HttpRequest', which is a link to its JavaDoc => do the same for 'resp' and 'reader' (i.e. what types reader has depending on the selected content type)