0.6.0

• [repocli] enable 2 retries by default, i.e. -r 2 (issue #12)
• [repocli] improve README (issue #13)
• [repocli] file is considered existing at destination if it has same file size and has later modification time at the destination (issue #7)
• [repocli] use buffered channel for the transfer queue so that total size and number of file counting are not blocked (issue #9)
• [repocli] add version sub-command (issue #10)