gemspec: add spec dependencies
gemspec add executables search

This is not actually needed yet but i might forget
gemspec: add atlas include path

To help find the headers for atlas
gemspec docs: better summary
Rakefile remove comment
ToDo: remove it
Remove io directory
add fftw::version::string
Add travis.yml for checking builds
Add fftw3 as git submodule
Tidy up logic in extconf
Use NMatrix includes
Simplify fftw3_nmatrix while it has yet to be tested
fftw3.rb: only use to require libraries