#708 : Use default locale encoding

Use locale endoding else this generate error in GU : as default time format can have non ascii char on non english locale, but using ascii in locale can generate UnicodeDecodeError: 'ascii' codec can't decode byte 0xc3 :

In PyBitmessage/src/bitmessageqt/__init__.py"
    l10n.formatTimestamp())

In PyBitmessage/src/l10n.py", line 81, in formatTimestamp
    return unicode(timestring, encoding)