Fix hash computing to be 16 characters exactly

Adding two 64bits hex via sumHex64bits method can return sometimes a 17-character hash.
You can have a 'retenue' when computing h_0 (line 159 in opensubtitles.js) just like computing h_1.
Opensubtitles API relies on 16 characters hashes (cf. http://trac.opensubtitles.org/projects/opensubtitles/wiki/HashSourceCodes#C2)