skype: make chdirs work more reliably between tests
skype: cleanup of trailing whitespace
+gitignore
skype: mention --plugindir quirk in HACKING

As skype support always gets built as plugin, option is required
for running tests without installing plugin to the system.
skype.test: do proper cleanup of skyped process despite possible exceptions, keep pexpect.log
skyped: use argparse instead of getopt for CLI