Add WRITE_ONLY option to open temp file with O_WRONLY

[timgimyee]

Fixes #18.

[timgimyee]

Not sure what this Travis CI error means:

xt/author/changes_has_content.t .. Dubious, test returned 1 (wstat 256, 0x100)
Failed 1/2 subtests 

Should I update the Changes file? Do I increment $VERSION?

[mohawk2]

I've squashed the two commits into one, and added a Changes entry, and force-pushed. That might need resolving against the other PRs recently merged, though.

[mohawk2]

I've updated the Changes and rebased it against the branch for #29 (on the assumption that will be acceptable - but that means you'd need to ignore the exports-related part of the diffs, or only review the last commit), and force-pushed. @xdg, does this look good to you?