Major merge in preparation for v1.8.0

[kadrlica]

This is a major update in preparation for tagging v1.8.0. Some major improvements include:

• Python 3 support
• Removal of pyfits dependency
• Better integration with healpix for sparse healpix files
• Expansion of the test suite
• Implementation of coverage fraction in masks
• Update and expansion of IMF functionality
• Relatively minor bug fixes

Relevant to #11, #15, #24, #36, #44.

[kadrlica]

@bechtol I'm not actually requesting a code review, but I thought you'd want to be aware of this.