Implement specialized 3D version
Update README.md
Make NFFT work on 32 bit systems

and add a 3D performance test
remove explicite Dim and type from NFFTPlan.

These were not necessary
remove the unsafe_load/store hack

Does not seem to give any peformance improvement with recent julia (0.3
prerelease)
add some documentation
improve test
Update README.md

The accuracy issues are due to the LUT. Increasing the LUT size improves the accuracy.