Improve downloading of dust maps

[henrysky]

This PR includes:

• Platform-independent downloads and decompressions by using python standard library
• Remove downloading options in setup.py
• Remove the need of specifying DUST_DIR environment variable, either use DUST_DIR if set else ~/.mwdust
• Download dust maps only when they are first called
• Remove function to change dustmap folder permission
• Allowing downloading all dustmaps all at once as well as test remote data file responds without downloading them
• README and setup.py tweaks

This PR allow platform specific compiled wheels to be used since downloading functions are moved from pre-installation to post-installation.

[jobovy]

Looks great! Here are some overall comments, I'll probably do another review once these are addressed.

[jobovy]

Okay, I tested this extensively against the previous downloads and everything seems to be working well, so I'll merge this. Thanks again, this is great!