created method flip_horizontal() for kivy.graphics.texture.Texture

Backing issue https://github.com/kivy/kivy/issues/944

Tested with code:

```python
#!/usr/bin/python
# -*- coding: utf-8 -*-
 
from kivy.app import App
from kivy.uix.gridlayout import GridLayout
from kivy.uix.widget import Widget
from kivy.graphics import Rectangle
from kivy.graphics.texture import Texture

class TestWidget(Widget):
	def __init__(self, **kwargs):
		super(TestWidget, self).__init__(**kwargs)
 
		# create a 64x64 texture, default to rgb / ubyte
		texture = Texture.create(size=(64, 64))
 
		# create 64x64 rgb tab, and fill with value from 0 to 255
		# we'll have a gradient from black to white
		size = 64 * 64 * 3
		buf = [x for y in xrange(64) for x in xrange(size/64) ]

		# then, convert the array to a ubyte string
		buf = ''.join(map(chr, buf))
 
		# then blit the buffer
		texture.blit_buffer(buf, colorfmt='rgb', bufferfmt='ubyte')	 

		with self.canvas:
			Rectangle(texture=texture, pos=(0, 0), size=(64, 64))
		
		texture.flip_horizontal()

		with self.canvas:
			Rectangle(texture=texture, pos=(64, 0), size=(64, 64))
 
class MyApp(App):
	def build(self):
		root = GridLayout(cols=2, padding=50, spacing=50)
		root.add_widget(TestWidget())
		return root
 
if __name__ == '__main__':
	MyApp().run()
```