Release 0.3.1

New feature in this release is --bind-try (as well as --dev-bind-try
and --ro-bind-try) which works like the regular versions if the source
exists, but does nothing if it doesn't exist.

The mount type for the root tmpfs was also changed to "tmpfs" instead
of being empty, as the later could cause problems with some programs
when parsing the mountinfo files in /proc.

```
Alexander Larsson (1 PR, 1 commit)
  Post-release version bump to 0.3.1 (#285)

Colin Walters (1 PR, 1 commit)
  Use "tmpfs" instead of empty string for mount (#278)

Patrick Griffis (1 PR, 1 commit)
  Add --bind-try options (#283)

chocolateboy (1 PR, 1 commit)
  Fix doc typo (#280)
```

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