process XML data before pretty-printing to trim whitespace

When receiving XML data that is already 'pretty' the current pretty printing process seems to add extra new line characters. I've modified the code to actually strip whitespace from the source input which seems to resolve this.

Input:
```python
>>> raw = '<a>\n  <b>blah</b>\n  <c>123</c>\n</a>\n'
>>> print raw
<a>
  <b>blah</b>
  <c>123</c>
</a>
```

Current:
```python

>>> print parseString(raw).toprettyxml(indent=' '*4)
<?xml version="1.0" ?>
<a>
    
  
    <b>
        blah
    </b>
    
  
    <c>
        123
    </c>
    

</a>
```

With patch:
```python
>>> processed = ''.join((x.strip() for x in raw.split('\n')))
>>> print parseString(processed).toprettyxml(indent=' '*4)
<?xml version="1.0" ?>
<a>
    <b>
        blah
    </b>
    <c>
        123
    </c>
</a>
```