include inner traceback in BuilderException

`BuilderException` tends to hide the problem which caused the actual exception. Some problems require digging through the code and manually tracing it, especially when the exception is occurring in a property callback.

This fixes that by adding the inner traceback to the exception message, which turns this cryptic exception:

````
 Traceback (most recent call last):
   File "extest.py", line 15, in <module>
     ''')
   File "/home/ryan/git/aeris2/kivy/kivy/lang.py", line 1630, in load_string
     self._apply_rule(widget, parser.root, parser.root)
   File "/home/ryan/git/aeris2/kivy/kivy/lang.py", line 1814, in _apply_rule
     e), cause=tb)
 kivy.lang.BuilderException: Parser: File "<inline>", line 3:
 ...
       1:
       2:MyLabel:
 >>    3:	text: '12'
 ...
 NameError: global name 'hello' is not defined
````

...into this:

````
 Traceback (most recent call last):
   File "extest.py", line 15, in <module>
     ''')
   File "/home/ryan/git/aeris2/kivy/kivy/lang.py", line 1630, in load_string
     self._apply_rule(widget, parser.root, parser.root)
   File "/home/ryan/git/aeris2/kivy/kivy/lang.py", line 1814, in _apply_rule
     e), cause=tb)
 kivy.lang.BuilderException: Parser: File "<inline>", line 3:
 ...
       1:
       2:MyLabel:
 >>    3:	text: '12'
 ...
 NameError: global name 'hello' is not defined
   File "/home/ryan/git/aeris2/kivy/kivy/lang.py", line 1808, in _apply_rule
     setattr(widget_set, key, value)
   File "properties.pyx", line 345, in kivy.properties.Property.__set__ (kivy/properties.c:3597)
   File "properties.pyx", line 377, in kivy.properties.Property.set (kivy/properties.c:4072)
   File "properties.pyx", line 431, in kivy.properties.Property.dispatch (kivy/properties.c:4665)
   File "extest.py", line 10, in on_text
     print hello
````