Avoid putting a space in empty comments

In our pipeline, empty comments in po files are generated without
a space after '#' and saving a po file using pofile lib generates
unwanted diff that need to be handle manually which is really annoying.
This pull request simply trim the comment line.

lib/po.js

@@ -20,12 +20,12 @@ PO.prototype.toString = function () {
 
     if (this.comments) {
         this.comments.forEach(function (comment) {
-            lines.push('# ' + comment);
+            lines.push(('# ' + comment).trim());
         });
     }
     if (this.extractedComments) {
         this.extractedComments.forEach(function (comment) {
-            lines.push('#. ' + comment);
+            lines.push(('#. ' + comment).trim());
         });
     }