TriGParserIterator class added providing a lazy iterator wrapper for TriGParser

[zozlak]

The new class serves a few purposes:

• Provides more PHP-native way of dealing with TriGParser results (while I guess every PHP user dealt with foreach probably not every dealt with passing a callable as a handler not to mention writing a lambda).
• Makes parsing in chunks easier as a user doesn't has to implement reading in chunks on his/her own. And for large inputs parsing in chunks is far more performant.
• Embeds support for streams.

The iterator is implemented in a lazy way (reads data only when they are requested and discards them when they aren't needed) which assures small memory footprint.

[pietercolpaert]

@k00ni Ok if I merge this? This looks good to me!

[k00ni]

@pietercolpaert I will assist you here and merge this.