Montage-wrapper

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- Fix bug that caused drizzle factor to be ignored in reproject, reproject_hdu,
and reproject_cube.

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- Added wrapper for mTileImage command. [14]

- Fix typo in name of mFixNaN command. [16]

- Ensure that reproject_hdu cleans up any temporary directories. [17]

- Make sure that temporary directories created by mosaic() are always removed. [19]

- The code base is now fully Python 2 and 3 compatible, and 2to3 is no longer used. [20]

- Use the astropy-helpers package. [21]

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- Disabled configuration file since it is not used. This gets rid of the
warning at import time.

- No longer check if Montage is in the PATH on install, but instead just on
import.

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- Added missing files required for installation

- Added a unit test for `mosaic`

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- removed unused files

- added documentation about setting custom MPI command

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- python-montage is now an Astropy affiliated package, and has been
renamed to montage_wrapper.

- improved performance of mosaic() by ignoring frames that don't overlap with
the requested header. Also improved exceptions in mosaic().

- new function ``reproject_cube`` to reproject a FITS cube slice by slice
(thanks to Adam Ginsburg).

- various bug fixes reported by users, and improved documentation.