Use int64_t when scanning integers, not int.

Summary: sizeof(int) == 4 means that we'll have overflow on large integers

Closes #3449

Reviewed By: @paulbiss

Differential Revision: D1657453

Signature: t1:1657453:1415121399:a71cf3326274f29caf3b825e7413bea332661cfd
Fixed xpath's php:functionString

Summary: Added CallerFrame before dom_xpath_ext_function_php's call to xmlXPathEvalExpression in case libxml2 needs to call back into HHVM.
The xpath query might contain ```php:functionString``` which triggers a call to any arbitrary php function.

Also fixed a bug where the arguments in an xpath php:functionString were reversed. For example, if the xpath was ```/foo[php:functionString('preg_match', '{$regex}',@​bar)]```, preg_match was being called as ```preg_match($bar, $regex)``` instead of ```preg_match($regex, $bar)```.

Closes https://github.com/facebook/hhvm/pull/4120

Reviewed By: @sgolemon

Differential Revision: D1656106

Signature: t1:1656106:1415132682:3423279074575c41ba3f39136ab2abd3f29ae232