Added USE_BSWAP_* options, "unroll-loops" explicit optimization

Now you can choose which bswap function will be used by #defining
USE_BSWAP_*.
Also added "unroll-loops" gcc optimization to vector decode and encode
functions where iteration count is constant (and equal to the length of
the vector used).
Cleaned 2 word vector processing, added "out.pbm" to .gitignore

The implementation is still very slow. :(
Can't use 16 byte vectors probably because the out image data is not 16
byte aligned