Update JSSpiderANE.as

asDoc comments added
all code formatting done in one style
unnecessary initialization with null removed (before constructor)
throwing an Error in constructor instead of a String becauese we prefer to have stack available
three lines of setting _context value in getContext reformatted to just one to make it more readable
expecting to get whatever possible instead of Error only in isSupported and in MethodHolder
a bit more concise and DRY approach in MethodHolder