Building at Mac OS X 10.10 Yosemite

To build at Mac I always receive errors within node-waf.

```
In file included from ../src/Database.cpp:22:
../src/Database.h:24:10: fatal error: 'sql.h' file not found
#include <sql.h>
         ^
1 error generated.
```

I have installed the unixODBC through `brew install unixODBC` and also tried `brew install unixODBC libodbc++`.

I have made those commits, don't know if they are suitable for all platforms and the `node-odbc` project itself.