Add suport for @Factory annotation

[edsonmedina]

Factories should not report new instances. (But maybe should complain about other behaviours (conditionals and statements))

So add support for an annotation that marks a class as a factory (@Factory).

[edsonmedina]

Let's ignore conditionals and statements for now and just focus on new

[edsonmedina]

No need for urgency, since #48 already addresses most cases.

[edsonmedina]

This has to wait for next php-parser release (same thing happened with @codeCoverageIgnore support)