Add same-codebase support for python 2.7.

When I opted for two codebases for python2/python3,
Send2Trash used C modules and it was easier to just have two
packages. With the ctypes version, supporting both python
versions becomes trivial and it's much more convenient to
merge them back into a single codebase.

I've only tested this code on Linux. I've converted plat_osx and
plat_win, but they only work theoretically.

send2trash/compat.py

@@ -0,0 +1,13 @@
+# Copyright 2013 Hardcoded Software (http://www.hardcoded.net)
+
+# This software is licensed under the "BSD" License as described in the "LICENSE" file, 
+# which should be included with this package. The terms are also available at 
+# http://www.hardcoded.net/licenses/bsd_license
+
+import sys
+if sys.version < '3':
+    text_type = unicode
+    binary_type = str
+else:
+    text_type = str
+    binary_type = bytes