Port C modules to Python 3.1 while keeping Python 2.4 compatibility

* py_stream_read and py_stream_write should operate on bytes because
  Subversion accepts raw data.
* Get the subvertpy modules to be compiled with Python 2 and Python 3 from
  the same source code.
* Move the definition of Py_ssize_t and T_PYSSIZET into py_fixup.h for
  consistency.
Add six.py
Use BytesIO instead of StringIO for Python 2 and Python 3.
Get six.moves as a submodule of subvertpy and add urllib.parse moves

* pathname2url, urljoin functions
Use subvertpy.six for urllib.
Adapt to Python 3’s dictionary view methods
import socketserver from subvertpy.six.moves
Use six.moves.xrange.
Use bytes for file data
Use byte strings rather than character strings in “delta” module

* Use bytearray() rather than a list of chunks when building up a buffer
Use six.py in marshall to handle type differences in Python versions
Use byte strings in marshall
Use new print() function syntax, with help from “2to3”
Use text character instead of byte for log action in Python 3
Document newer Python dependency
Write bytes to stdout in subvertpy-fast-export and example
Merge branch 'common' into py2.6

Conflicts:
	subvertpy/ra.py
	subvertpy/tests/test_client.py
	subvertpy/tests/test_wc.py