Merge pull request #21 from ChrisBeaumont/master

Small tweaks and bugfixes
Re-implement set_[xy]label
WCSAxesSubplot class.
Merge pull request #23 from ChrisBeaumont/axis_label

Re-implement set_[xy]label
Merge pull request #22 from ChrisBeaumont/tight_axes

WCSAxesSubplot
Modified constructor of WCSAxes class to allow overriding transData

- useful for doing flexible things with the rendering
Merge pull request #13 from ejeschke/pass-transdata

Modified constructor of WCSAxes class to allow overriding transData
Refactor transformation classes to better support slicing of WCS
Generalize find_coordinate_range to arbitrary number of coordinates
Generalize CoordinatesMap to arbitrary number of coordinates, with slicing
Transformation default is now pixel to world since it is always defined
Add slicing and arbitrary number of coordinates to top-level WCSAxes class
Refactor tick rotation calculation to depend only on pixel-to-world transformation.
Add a new experimental mode for drawing grid lines using contour - this avoids ever needing a world-to-pixel transformation which is ill-defined if distortions are present or when slicing a cube.
Pass slices to reset_wcs
Fix order of transformations following refactoring
Fix bug in tick rotation
Fix ticks for origin='upper'
Make grid_helper an option for grid
Fix plotting for cubes when grid is not requested