Improved decoding performance:

When recovering missing source symbols, one can use the same function
used for encoding repair symbols from intermediate symbols, only applied
to ISIs less than K (associated to source symbols). This results in an
improved performance.
Improved the javadoc and polished the code:
- removed "core" package in order to better indicate the main package
net.fec.openrq
- added javadoc for main, encoder, decoder and parameters packages
- improved the code a bit everywhere
Improved ceiling division function.
Added generic encoding packet generation
Added iterator code to encoder classes