Rename method to set_source_text.

There's a difference between 'text' and 'source text', where the former
is a more user-friendly representation of a todo, while the source text
is the raw text from the todo.txt file.

Reflect this difference in the method name to avoid confusion with the
'text' method.

TodoBase.py

@@ -23,7 +23,7 @@ class TodoBase(object):
         self.src = ""
         self.fields = {}
 
-        self.set_text(p_src)
+        self.set_source_text(p_src)
 
     def tag_value(self, p_key, p_default=None):
         """
@@ -139,7 +139,7 @@ class TodoBase(object):
         """
         return self.text(True)
 
-    def set_text(self, p_text):
+    def set_source_text(self, p_text):
         """ Sets the todo source text. The text will be parsed again. """
         self.src = p_text.strip()
         self.fields = TodoParser.parse_line(self.src)

TodoList.py

@@ -136,7 +136,7 @@ class TodoList(object):
 
             if todo:
                 new_text = todo.source() + ' ' + p_string
-                todo.set_text(new_text)
+                todo.set_source_text(new_text)
 
     def projects(self):
         """ Returns a set of all projects in this list. """

test/TodoBaseTest.py

@@ -235,11 +235,11 @@ class TodoBaseTester(unittest.TestCase):
 
         self.assertEquals(todo.src, "x 2014-06-13 Foo")
 
-    def test_set_text(self):
+    def test_set_source_text(self):
         todo = TodoBase.TodoBase("(B) Foo")
 
         new_text = "(C) Foo"
-        todo.set_text(new_text)
+        todo.set_source_text(new_text)
 
         self.assertEquals(todo.src, new_text)
         self.assertEquals(todo.priority(),'C')