Add supported protocols based on libcurl version

Extend the supported protocols based on the major and minor versions of
the libcurl development libraries in the system.

RHEL6 is based on 7.19 and hence does not contain some of the protocols
added in bd5c53b1.

Signed-off-by: Carlos Aguado Sanchez <carlos.aguado@epfl.ch>
Add options to handle HTTP user authentication

Add the CURL options for managing the username and password. In
particular:
- CURLOPT_USERNAME
- CURLOPT_PASSWORD

This will allow the usage for HTTP authentication schemes such as basic
HTTP authentication or GSSAPI/SPNEGO.

Signed-off-by: Carlos Aguado Sanchez <carlos.aguado@epfl.ch>
Amend registration logic of supported protocols

Decide whether to support a protocol based on its availability in
the specified libcurl implementation.

Signed-off-by: Carlos Aguado Sanchez <carlos.aguado@epfl.ch>
Merge pull request #11 from caguado/fix/authn

Add authentication options and build in RHEL6

(Merging this, since no one complained)