add check for libaio before downloading a distribution

- this check is restricted to mysql 5.5.4+ per mysql bug 60544.
  dbsake will attempt to locate aio via ctypes.util.find_library
  and abort if it is not available
add --skip-libcheck option to mysql-sandbox command

- resolves issue #3
handle mysql versions more robustly

- MySQLVersion.from_string now handles '-<tag>' in version names
  better to support 5.7.3-m13 or other tags that may come from
  other version sources
don't completely swallow stderr from mysqld --version

- when verifying the mysqld version, if this errors out, emit
  the stderr.  This mostly to fix issue #3, where missing libaio
  causes mysqld --version to fail, and the cause wasn't immediately
  obvious.
remove stale commented code from mysqld_version()
Merge branch 'feature/libaio_check' into develop