Initial checkin of SBT build files.

Also ignore IntelliJ Idea project files.
Particles and 2D grid-based acceleration structure.

Particles currently have properties of mass, position and velocity.
Particles are stored in a ParticleBag, which employs a 2D grid-based
acceleration structure to enable fast neighbouring-particle look-ups.
ParticleBags are constructed by adding particles to a
ParticleBagBuilder and then calling the result method.
Simple Quintic smoothing kernel.
Merge branch 'release/v0.0.1'