Use cpr library to interface curl

[prudhomm]

@vincentchabannes I think cpr would help us simplify the code that handles downloads and uploads.

• Basic auth is available (user/pass and oauth)
• GET/POST and multiple parts

also it uses modern cmake

what do you think ?

[prudhomm]

I have integrated cpr into feelpp in contrib, it should be pushed very soon to github.
I am currently using to generate multipart post request and sending both json and data files