fix tsp internal variable type to double.

This fix will support tsp distance variables < 1.0 case (#159).
I confirmed that all test passed on Ubuntu12.04x86/Windows7x64(winnie-rc1)/MacOSX.

The reported query and its result was as follows.
<pre>
pgrouting-workshop=# SELECT * FROM pgr_tsp('{{0,0.0239288164765725,0.0257979665805287,0.031582558789943},{0.0239288164765725,0,0.0211958716829549,0.0379751989289084},{0.0257979665805287,0.0211958716829549,0,0.0180511085745426},{0.031582558789943,0.0379751989289084,0.0180511085745426,0}}'::float8[], 1);
 seq | id 
-----+----
   0 |  1
   1 |  3
   2 |  2
   3 |  0
(4 rows)
</pre>