Issue #26: New integer tests

https://github.com/alobbs/libhpack/issues/26

There were several important cases that were not tested by *integer_test.c*

Added tests:
- *decode_too_big_for_int*: Decodes a number > INT_MAX
- *decode_too_big_for_uint*: Tries to decode a number > UINT_MAX
- *decode_too_big_for_ulong*: Tries to decode a number > ULONG_MAX
- *en_decode_max_uint_5bits*: Encode/Decode INT_MAX with 5 bit prefix
- *en_decode_2nd_byte_0*: Encode a number with 1st byte 1s and 2nd 0s
- *decode_too_many_zeros*: Tries to decode a number with too many zeros

Most of these tests fail with the current integer encoding/decoding
algoritm.