Streamline destroy logic

Now will always validate snapshot name once (rather than twice in
most situations).
Reorder ValidTTL tests
Add support for 'forever' TTL

Fixes #49
Make in-program help match manpage better
Small comment fixes
Protect against leading zeros in TTLs

sh, in its infinite legacy wisdom, treats numbers beginning with zeros
as octal, which can lead to unintended side effects. Rather than check
and convert throughout the code-base, it is easier to just reject TTLs
beginning with zeros.
Small formatting and language fixes for manpage
Replace remaining echo with printf

echo is considered harmful. http://www.etalabs.net/sh_tricks.html
Make "Unsupported OS" error more informative
Update readme