Catch concurrent snapshots

Curator now shows a proper error in the event of concurrent snapshots

closes #177
Fix logging `_, a, l, l` with --all-indices
Updated Contributors & Changelog
Fix logging to work with stack traces
Add 2-digit years (lower-case y)

This adds 2-digit years as an acceptable pattern.  This is lower-case 'y' vs. upper-case (4-digit).
closes #209
Fix #214 by adding check if index is closed, before adding to alias
Updated CONTRIBUTORS & Changelog

closes #214
Made install_requires variable based on python version

newer python versions have argparse builtin
Updated CONTRIBUTORS & Changelog