Fix #26: New VLQ integer encoding/decoding

https://github.com/alobbs/libhpack/issues/26

Encoding would encode numbers greater than INT_MAX.
Now it can enconde up to UINT_MAX.

Decoding would have trouble with:
- Numbers greater than INT_MAX --> Return negative number
- Numbers greater than UINT_MAX
- Would process all bytes of a stream with many zeros

According to the draft if we encounter too big a number or too many
zeros in the encoding we should consider this an error. Now it does.