Encode issue string as UTF-8

Was getting a `TypeError: bad argument type for built-in operation` when running `:Gissues`.

After some tracing I found it was cause `isssuestr` was encoded as default python unicode (UCS-2) and my vim wanted UTF-8. Thus, I think we need to re-encode the string as such.

(I'm on linux w/ vim+tmux)
Merge pull request #25 from mikeycgto/patch-1

Encode issue string as UTF-8