Example deallocation

Original has a bug in make_hash; the digest its returns is a stack variable.  If you do d=make_hash then use the stack and then try to deference d, it may be corrupted.  I've changed it to alloc the hash but the caller must subsequently free() it.

Your library works fine for me with Arduino 1.0.6 but when I switched to 1.5.8 the example no longer gives the correct MD5 due to memory corruption. mrparp has fixed this issue in his fork and I have updated the examples to deallocate.