Fix #20.

There's no need to append an empty string to preedit.

Hopefully this won't bring some side effect, since we don't ship any
macro file by default.

This resolves #13.

This fixes #10.

Disable spell check by default to workaround a firefox issue. Usually
firefox's feedback is slow on input method issue. Workaround from
ourside would be a better solution.

Emm. No idea why this is prevented. setIM itself does support
everything, now viqr and ms will work with input method.