Tidy up old submodules and make some new ones.

* Removed the ssw environment stuff
* removed sunpy.coords
* added a new sunpy.physics module
* moved the differential rotation code to sunpy.physics.transforms.differential_rotation
* removed the modified versions of astropy's sphinx extensions as they are now in astropy
* removed the units placeholder as it is taken by astropy.units
Fix tests
Fix the doc build
remove the pavement script
Add changelog entry [ci skip]

Conflicts:
	CHANGELOG.md
more doc fixes
Remove benchmarks dir and mac_install.py
No caps in module titles
Merge remote-tracking branch 'upstream/master' into 0.5_tidyup [ci skip]

Conflicts:
	CHANGELOG.md
Merge pull request #1004 from Cadair/0.5_tidyup

Make a new sunpy.physics module and remove some not used submodules.