Implement initial differential_rotation routines.

This is a major rebase and squash, below is a list of all the old
commits.
adding in rot_xy

implementing rot_xy.pro

sun_pos.py nearly done, on the way to rot_xy.py

sun_pos.py working

filling in more of pb0r

working on pb0r function

fixed error in sun_pos; error in pb0r to do with arctan?

fixed pb0r - now gives results the same as the IDL version down to the 7th decimal place approximately.

trying to track down the bug in rot_xy.py - maybe one of the WCS files?

seemingly fixed rot_xy; results now very similar to IDL's rot_xy, but still a little different.

convert_hcc_hg appears to be the correct inverse of convert_hg_hcc; in IDL, arcsec2hel appears to be the inverse of hel2arcmin.  However, the values calculated by convert_hcc_hg and arcsec2hel are different.

added some notes

completing rot_hcc hopefully, and putting that functionality into Mapcube

tidying up white spaces

tidied up rot_hcc calling, and added a spacecraft option

changing the rotation function

fixed the solar rotation function rot_hpc

minor changes

rename p0br

attempted fixes to post-staging merge

remove prints

wcs modifications

more small tweaks to util

small changes

Fix mapcube regression

Fix diff_rot tests by using almost equal

Fix diff_rot tests by using almost equal

Tweaks and 80char lines

Fix a kwarg bug and change some test values!

revert mapcube after rebase.

revert wcs after rebase

move the test file into the new location
Merge pull request #1153 from sunpy/master

update differential_rotation branch
first small edits
turning some functions to internal, and making sure that these functions return astropy Quantities.
made the true obliquity an astropy Angle Quantity
more edits to differential rotation
implementing astropy units in the rotation code
first working rot_hpc using existing wcs transformation code
improving the documentation
adding in tests

adding in tests
creating a mapcube solar derotation function
added in a function to test if the input is an astropy pixel Quantity, as this function is explicitly used in many different functions.
small change
small changes
replaced pixel testing function and added arcsec testing function
add in the rotation function
pulled out the solar differential rotation function
expanding tests
Merge branch 'differential_rotation' of https://github.com/sunpy/sunpy into ji_diff_rot