Write all languages to the twine strings file.

We used to only write translations to the strings file that did not
match the developer language. It was done this way to try and reduce
space in the strings file and to get en-UK to pick up en changes.

This turned out to be the wrong decision. It's better to explicitly
ensure that all strings are translated in all languages, and to not
automatically pick up en string changes in other languages.

This fixes issue #29.
Add test for multiline strings in .po files
Fix parsing of .po files for multiline msgstr

It's valid for a msgstr to be broken over multiple lines, all of which
are enclosed with ".  To handle this, no longer iterate over the lines within
an item but rather apply the matching to the entire item.
Added gitignore
Modified gettext formatter to print out sections as .po comments
Updated git ignore
Fixed gettext test
Add Joseph Earl to the list of contributors.