Updating .gitignore to exclude the build-tmp directory

build-tmp/ directories are created by the sublime-processing package
Fixed the .gitignore directive for excluding build-temp directory
Refactored Sphere and SphereCell; fixed calls to ellipse()

Refactored Sphere and SphereCell to their own .pde files; fixed calls
to ellipse to accurately draw using their radius properties (they
previously called ellipse() with the radius property passed in as the
diameter property)
Removing class declarations for Sphere and SphereCell from main .pde
Updated gitignore to exclude web-export directory generated by Processing
Added gravitational attraction among spheres in a given cell

Spheres between different cells do not attract each other; as well, a
constant force tries to push each sphere back into its cell if it
leaves the cell.
Updating gitignore
Fixed gitignore -- again
Removing files that are now being ignored by gitignore