Support writing a po back

[singalen]

After parsing, some could need to write back a po file.
That is, including header and preserving comments.

[ngocdaothanh]

Can you implement it (I think it takes about 1 hour) and give me a pull request?

[singalen]

Will try to. Though the estimation is pretty optimistic given I need to learn some of Scala first.
Thank you again!

[ngocdaothanh]

This feature is convenient when we want to merge multiple .po files, then write out the merged one:

val po4 = po1 ++ po2 ++ po3

[ngocdaothanh]

See #14.
Now you can easily write a po back.

In v1.7, there's no code to merge po files yet.