Add cURL source type (addresses #238)

At this moment only JPEG (single image) and MJPEG (stream) are supported.
Digest authentication and other HTTP/1.1 features are fully supported in cURL.

The cURL docs say not to put the username and password in the URL field, but store them in separate fields, so 2 new fields need to be added to the Monitors table:
```
ALTER TABLE `Monitors` ADD `User` VARCHAR(32) NOT NULL AFTER `SubPath`;
ALTER TABLE `Monitors` ADD `Pass` VARCHAR(32) NOT NULL AFTER `User`;
```