Support multiple backends

[hkupty]

Curl does the job but:

• involves communicating through stdout, therefore:
  • it has only one channel to transmit the information
  • To get HTTP statuscode, we'll have to parse the output
• It's a synchronous system call:
  • Can be made asynchronous but I guess that'll make things a bit more complicated;
  • using jobstart might be an option, but then maybe it'll need to concat the output before parsing.

Supporting alternative backends will allow for:

• gradual migration;
• testing different approaches;
• ensuring a stable backend exists as alternative;
• falling back to curl if nothing is available;
• supporting other platforms (i.e. Windows)