Murmur optional alternative

This patch adds the option to compile the library without murmur as hashing algorithm. If compiled without murmur, the `ht_init()` function signature is changed and gets function pointers to the hashing functions. 

I was not able to test the lib, as the tests do not run (described in #7). It compiled fine for me, except the one error when compiling without murmur, that main.c fails because the function signature does not match anymore. This is a wanted behavior, as we are now able to compile the lib into own applications by including the source directly (not over a lib*.so) and set our hashing functions on runtime.

The lib provides now per-hashtable instance hashing functions.

Tell me what you think about it, comments and improvement requests heavily welcome!