Pushbullet use type "link" when message_uri is set
Add explicit proxy support when using libcurl.

The appropriate *_proxy environment variables aren't always detected properly
by libcurl. This allows manually setting the proxy when this happens, and
disabling SSL certificate verification if the proxy requires it.
Update README
Trailing whitespace
Merge pull request #112 from creesch/patch-1

Pushbullet: use type "link" when message_uri is set
Fix #114: revert proxy change that breaks w/o libcurl