Add proxy support for both requests and curl mode

It's now possible to specify which proxy should be used for each account.
Requests mode still needs [1] to be fixed.

[1]: https://github.com/shazow/urllib3/issues/170

i.e.,
"accounts": {
  "GitHub": {
    "base_uri": "https://api.github.com",
    "https_proxy": "http://localhost:3128"
    ...
  }
}
...
Add flag to force curl mode

This is required in order to use a proxy until requests is fixed.
Detect command execution errors

This is required in order to detect and notify commandline errors (i.e., curl)
Fail fast when response is empty

This might happen when CurlSession.request fails
Update README with information about using a proxy
Merge pull request #42 from astratto/addProxySupport

Add proxy support (2nd attempt)