core: Text introduce justification of text. Introduces `left_justified`,`right_justified`, `center_justified`

I'm testing with the following code, with and without markup.

```
#-*-encoding:utf8-*-
from kivy.uix.gridlayout import GridLayout
from kivy.lang import Builder


root = Builder.load_string('''
BoxLayout:
    padding: 22
    padding: 10
    orientation: 'vertical'
    Label:
        id: lbl_just
        valign: 'middle'
        markup: True
        text_size: self.size
        halign: 'left'
        text:
            'Thy father was delighted and cried out to the servant,\
 ‘Give him a hundred and three gold pieces with a robe of honour!’\
 The man obeyed his orders, and I awaited an auspicious moment,\
 when I blooded him; and he did not baulk me; nay he thanked me and\
 I was also thanked and praised by all present. When the blood-letting\
 was over I had no power to keep silence and asked him, ‘By God, O my lord,\
 what made thee say to the servant, Give him an hundred and three dinars?’;\
 and he answered, ‘One dinar was for the astrological observation, another for\
 thy pleasant conversation, the third for the phlebotomisation, and the\
 remaining hundred and the dress were for thy verses in my commendation.’”\
 “May God show small mercy to my father,” exclaimed I, “for knowing the like of\
 thee.”[1]\\n\\n'\
 'And indeed thou shalt never find a man better versed in affairs\
 than I, and I am here standing on my feet to serve thee. I am not\
 vexed with thee: why shouldest thou be vexed with me? But\
 whatever happen I will bear patiently with thee in memory of the\
 much kindness thy father shewed me." "By God," cried I, "O thou\
 with tongue long as the tail of a jackass, thou persistest in\
 pestering me with thy prate and thou becomest more longsome in\
 thy long speeches, when all I want of thee is to shave my head\
 and wend thy way!"'
    Spinner:
        size_hint: 1,  .1
        text: 'left'
        values:
            ('left', 'right', 'center', 'right_justified', 'left_justified', \
            'center_justified')
        on_text: lbl_just.halign = args[1]

    
''')

if __name__ == '__main__':
    from kivy.base import runTouchApp
    runTouchApp(root)

```
![snapshot1](https://f.cloud.github.com/assets/788023/239645/e795f36e-88b3-11e2-816a-aa6fd623af20.png)