CHANGELOG: Update with summaries of recent changes

This documents the following changes:
* f01eac2 (email: Make path to sendmail configurable, 2013-02-15)
* 1c97270 (email: Attempt .as_string() if BytesGenerator.flatten()
  fails, 2013-02-16)
* e08e198 (email: Decode headers when checking .as_string() flatten
  fallback, 2013-02-17)
* 3adef87 (config: Use extended interpolation in Config, 2013-02-21)

Signed-off-by: W. Trevor King <wking@tremily.us>
Bump to version 3.2

Signed-off-by: W. Trevor King <wking@tremily.us>