Add custom request method

This should take care of issue #5.

Basically adds a new `custom_method( STRING )` function that accepts the method you would like to send instead of the standard GET/HEAD/POST.

After running it you still have to call the normal get/head/post methods since that defines which code path libcurl will take per the docs.

See CURLOPT_CUSTOMREQUEST on http://curl.haxx.se/libcurl/c/curl_easy_setopt.html

Example:

```ruby
# send a PURGE request
# libcurl will use still perform a normal get()
# but use PURGE as the request method
curl.custom_method("PURGE");
curl.get("http://example.com/uri");
```