[Pyublas] add bool converter

[Neal Becker]

Works for me:

diff --git a/src/wrapper/converters.cpp b/src/wrapper/converters.cpp
index eea9d2a..c6d0466 100644
--- a/src/wrapper/converters.cpp
+++ b/src/wrapper/converters.cpp
@@ -457,6 +457,7 @@ namespace
 
 void pyublas_expose_converters()
 {
+  expose_converters<bool>();
   expose_converters<npy_byte>();
   expose_converters<npy_ubyte>();
   expose_converters<npy_short>();