Fixed multiple occurences in extraction leading to overwritten desc-valu...

When multiple translations occur with the same id, then the value specified in the Desc-annotation gets incorrectly overwritten by the future scanned occurences. This is because the localeString is overwritten in subsequent merge-operations.

I have moved the overwriting of localeString so it is dependent on the desc-attribute of the newly extracted message that is merged in. If the newly extracted message has a Desc-attribute then this will overwrite the Desc-attribute and the localeString attribute synchronously. If the Desc-attribute is missing from the new message, then both attributes will be ignored.

Should fix issue #86