Fix 64-bit compatibility compiler warnings

Fix compiler warnings in case of sizeof(unsigned)!=sizeof(size_t)
 * HASH_FIND_STR is tested
 * HASH_REPLACE_STR is not tested
 * no (additional) cast required for HASH_ADD_STR since already done in HASH_ADD_KEYPTR
Merge pull request #35 from tbeu/patch-1

Fix 64-bit compatibility compiler warnings