Adds surrounding " quotes to strings with markup.

[murraycu]

Unescaped markup is apparently normal in Android strings.xml files, according to:
http://developer.android.com/guide/topics/resources/string-resource.html

For instance, my values/strings.xml can have this:

<string name="about_text2">This android app was developed by <a href="http://murrayc.com/">Murray Cumming</a></string>

And my values-de/strings.xml can have this translation:

<string name="about_text2">Diese Android App wurde von <a href="http://murrayc.com/">Murray Cumming</a> entwickelt.</string>

However, when I export and import with a2po, that translation changes to:

<string name="about_text2">"Diese Android App wurde von "<a href="http://murrayc.com/">Murray Cumming</a>" entwickelt."</string>

Notice the extra quotes around the whole string and around the markup. Android seems to ignore them, and they don't appear in the .po file when I export again, but it's pretty strange. I'd feel better if I knew that it's really legal and correct.