Raise a visible exception for ConnectionError
Add proxy support

Fixes issue #31.

It's now possible to specify which proxy should be used for each account.

i.e.,
"accounts": {
  "GitHub": {
    "base_uri": "https://api.github.com",
    "https_proxy": "http://localhost:3128"
    ...
  }
}
...
Merge pull request #40 from astratto/addProxySupport

Add proxy support