Unwind V8 execution context on fatal error, refs #87

This is yet a first hack to prove applicability.  Currently
unwind environment is held in a global variable, i.e. solution
is neither thread safe nor reentrant yet.
Use V8JSG instead of global variable.
Store error message and re-throw with original type & message
Handle fatal errors even if executeString is used recursively
Code folding marks and comments
Merge pull request #91 from stesie/hack-fatal-error-unwind

Handle PHP Fatal Errors inside JS->PHP callbacks