Add execution timeout

[asmblah]

Infinite loops in the browser can lock up the UI so are a big no-no. Uniter should support the set_time_limit(...) function in order to handle this scenario.

The default of 30 seconds in the Zend engine is too high for in the browser: a default of 1 second should be enforced there.

[asmblah]

Added initial draft of PHP execution time limit support in feature/execution-timeout branch.

• Currently naively hooks 'do...while' and 'while' loops with an expiry check for every iteration
• Could be improved to only check every N iterations
• Still need to add support for other loop types, eg. 'for'