Avoid UnicodeEncodeError. (#544)

* Fix wrong README format.

* [FIX][base_import_match] Avoid UnicodeEncodeError.

When the model or field you chose was translated and had some non-ascii
character, you got an error like this: `UnicodeEncodeError: 'ascii' codec can't
encode character u'\xed' in position 15: ordinal not in range(128)`.

Now, using unicode strings, that won't happen again.

* Do not require a hidden field.

* Further unicode protection, add ondelete clause.

base_import_match/views/base_import_match_view.xml

@@ -20,7 +20,9 @@
                         <tree  editable="bottom">
                             <field name="field_id"
                                    options="{'no_create': True}"/>
-                            <field name="match_id" invisible="True"/>
+                            <field name="match_id"
+                                   invisible="True"
+                                   required="False"/>
                             <field name="model_id" invisible="True"/>
                             <field name="conditional"/>
                             <field