[WIP] HHVM support & compatibility

Hi,

this PR adds a support for HipHop Virtual Machine (aka HHVM).

I am not yet proposing this as a definite solution, there are some things I do not like. But some of them are due too simple/bad design of Tester leading to low abstraction, therefore bad executeble-specific behavior.
- [x] `TESTER_HHVM` - this env variable is needed for detection of HHVM now, how could we make Tester live without it? Maybe create a factory for executable and inspect binary there?
- [x] some places with conditional checking for PHP/HHVM which I do not like, but without larger Tester refactoring, hard to deal with.

I'm open to discussion, I would love to see HHVM support here. This also blocks Nette compatibility - without Tester support, there is no way to propose Nette as "popular framework" to be added to HHVM's parity suite.