Fix #35: Libchula's *buf* should be unsigned

https://github.com/alobbs/libhpack/issues/35

After dettecting in issue 33 that having *chula_buffe_t -> buf* as *char* created issues with bitwise operations, it was decided that:
>chula_buffe_t -> buf should be either uint8_t or unsigned char.
>The current signed type is somehow error prone for most common cases of the class.

This commit changes to *uint8_t* chula's *buf* field and also adds all necessary casts in functions.

Also in file *buffer.c*, function *chula_buffer_cmp_buf* a *strncmp* has been changed to a *memcmp*
Original: `return strncmp (A->buf, B->buf, B->len);`
Now: `return memcmp (A->buf, B->buf, B->len);`