Added custom HTTP method functionality

A small addition to add the ability to use arbitrary HTTP verbs, in case you might need to.

The methods are in line with the existing format, except with an additional string parameter representing the request method.

Added methods are:

* Network.Wreq.customMethod
* Network.Wreq.customMethodWith

As well as exporting prepareMethod from

* Network.Wreq.Internal.prepareMethod


Examples of use are:

~~~
>>> r <- customMethod "PATCH" "http://httpbin.org/patch"
~~~

~~~
>>> let opts = defaults & redirects .~ 0
>>> r <- customMethodWith "PATCH" opts "http://httpbin.org/patch"
~~~

Let me know if I've taken any missteps here as I'm not super familiar with the library, I added this method as I have a web-service in development that uses some custom HTTP methods and wanted to test it out with wreq :)