Add curl module

Hello guys,

This PR adds curl plugin support.

But it also serves as a PoC of plugin configuration combining both defined resources and parameterized class for hiera support.
I wrote the defined types that can be called directly, and I also used create_resources() to allow plugin configuration via Hiera.

Please give me your feedback on this module as well as on this pattern, if you like it I'll modify #138 to use that pattern

I'm already using this code in production by using the defined resources, I do not use the hiera-style invocation, but it seems to works