Rename ghost module to overlap or some other term

[mrocklin]

This term is probably not the clearest. We should continue replacing it with something else, like overlap.

cc @jakirkham

[jakirkham]

...or halo. ;) Though maybe overlap is clearer.

@ogrisel used another term that was good, but have since forgotten. Maybe he remembers?

[jakirkham]

Also @jni may have thoughts here ;)

[jni]

in terms of clarity, imho, overlap > halo >> ghost.

Halo makes sense if I think about it, but instinctively my mind goes to Beyoncé. =P

[mrocklin]

This would require the following:

1. Changing the function names within the ghost module
2. Renaming the ghost module to overlap
3. Changing the documentation and API documentation
4. Including a deprecated ghost module with deprecated functions that warn and then point to the new ones