HHVM: default config is disabled by -n [Closes #159]; minimal version is 3.3.0
HHVM: hide stack trace for uncatched exceptions [Closes #160]

Without this option, Fatal Error stack trace is printed even the set_exception_handler() is used (e.g. AssertException printing).
tests: workaround for missing Closure::bindTo() in HHVM
tests: Runner.multiple-fails.phpt partial HHVM fix
tests: Runner.multiple-fails.phpt fix [Ref #162]
Merge pull request #165 from milo/pull-hhvm-fixes

HHVM fixes - tests pass hunting
Tester doesn't need JSON extension [Closes #171]
Tester doesn't need iconv extension
Merge pull request #172 from milo/pull-remove-json

Remove JSON and ICONV dependency
tester: default thread count is 8

The reason is memory saving.
Based on nette/tester#165 discussion.