MultiCurl: Retrieve multiple pages simultaneously

This class can be used to retrieve multiple pages simultaneously from remote Web servers.

It uses PHP Curl extension to establish one or more HTTP requests that may be executed in parallel.

The class supports:

- Only a limited number of requests are executed in parallel, so exceeding requests are delayed until prior requests are finished
- Restrict the size of retrieved data to a given limit
- Set common CURL options for all requests
- Set separate CURL options for different requests when necessary
- Use callback events for response result processing.