Document (web response) and HTTP fetching

* doc/Makefile.am: add new examples
* doc/ex_http_simple_get.scm: new example
* doc/ex_http_simple_get_utf8.scm: new example
* doc/ex_http_web_response.scm: new example
* doc/guile-curl.texi: more documentation
* doc/ex_http_header_charset.scm: example removed