Adding enforcement of the wanted graphics system GL/GLES
- GRAPHICS=GL/GLES
use a blank 1x1 texture for empty line. closes #1913
fixes for tito for pep8 and using one variable
Fix setting exit_on_escape atribute on on_keyboard method (#1967)
Allow setting source to empty string to clear text, if it wasnt empty before.
Python3 and pep8 fixes for 3d rendering example
Merge pull request #1987 from inclement/3drendering

Python3 and pep8 fixes for 3d rendering example
created method flip_horizontal() for kivy.graphics.texture.Texture
Add cursor_color property for TextInput
update style.kv
use a more natural method to check if self._text is unicode.

this is to handle the following case
```
class LabelText(unicode):
    def do_something():
        return repr(self)

Label(text=LabelText("some Message!"))
```
which would cause kivy to crash prior to this fix
Merge pull request #1998 from joranbeasley/core_text_fix_py2_gettext_for_unicode_inherited_classes

use a more natural method to check if self._text is unicode.
Added keep_within_parent splitter option
Added parent proportion tracking to splitter
Splitter: Added rescale_with_parent property
Splitter: Corrected default property values
Splitter: Added versionadded annotations
Splitter: Property default changes for consistency
Fix swap of errorvalue with errorhandeler in the doc
graphics: add default texture mapping for line (should not change anything for everybody by default, but help to map custom texture onto it.