fix several bugs:

* fixed sint64 decoding (missing zigzag encoding)
* fixed fixed64 returning types. it returns long (or double)
* add #16 test cases.

still exists larger value problems regarding PHP's implicit type conversions.
I'll adjust numeric basic getter / setter method which doesn't regard floating point values.

SETTER: long or string (double may cause positive / negative infinite problem.)
GETTER: string (probably I'll add getAsLong or something methods)