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 PR in production by using the defined resources, I do not use the hiera-style invocation, but it seems to works