Improve performance for large files

xmltodict becomes slow when you have an XML file with large texts.
Enabling the parser.buffer_text option dramatically increases performance.
Benchmark below. Unittests still work.

Code used to benchmark:

import xmltodict
import time
xml = "<root>" + ("a"*70+"\n")*10000 + "</root>"
s=time.time()
x=xmltodict.parse(xml)
print(time.time() - s)

 # 19.9860811234 seconds without buffer_text
 # 0.059289932251 seconds with buffer_text
 # So 300 times faster