Fixes, primarily to documentation

* Installation instructions now have a command, using `pip`, so that users can properly upgrade their SunPy (specifically, recompiling `Crotate`) (see #391)
* Windows installation instructions have been updated after testing with the latest Python(x,y) (no need to separately install `pip` or to work around a compiler-configuration bug)
* `Crotate.affine_transformation` did not have a proper docstring (no function signature, which is needed in C-API, or a parameter list), so I created something to start with, but it needs to be fleshed out
* Unrelated to documentation, but I noticed that the removal of PIL checking from `sunpy.util.sysinfo.py` (see #379) was inadvertently reverted by a later merge, so I removed it again.