Don't convert cell value to string if it's already a string

When trying to write an ODS file, containing a cell with a string value, using the following code (adapted from http://simple-odspy.sourceforge.net/?q=node/6)
```
from simpleodspy.sodsspreadsheet import SodsSpreadSheet
from simpleodspy.sodsods import SodsOds

t = SodsSpreadSheet()

t.setValue("A1", "Set one cell")

tw = SodsOds(t)
tw.save("test.ods")
```
I am getting the error message
```
Traceback (most recent call last):
  File "test.py", line 9, in <module>
    tw.save("test.ods")
  File "/usr/lib/python3.4/site-packages/simpleodspy/sodsods.py", line 382, in save
    tc.addElement(P(text = str(escape(c.text), 'utf-8')))
TypeError: decoding str is not supported
```
I fixed this for me by changing `simpleodspy/sodsods.py` like you see in the pull request. Basically I skip the conversion to a string if it is already a string. I don't know if this affects other parts of the library. In my use case, I only had cells with string values. Please review and merge, thanks!