create cyclus.h. added missing header install

Projects using cyclus core can now just `#include "cyclus/cyclus.h"`.  Note that thanks to libxml2 and coin, if anybody including cyclus.h this also has to do some compiler flag gymnastics akin to this:

```
-I/usr/include/coin
-I/usr/include/libxml2
-I/usr/include/libxml++-2.6
-I/usr/lib/libxml++-2.6/include
-I/usr/include/glibmm-2.4
-I/usr/lib/glibmm-2.4/include
-I/usr/include/glib-2.0
-I/usr/lib/glib-2.0/include
```

Long live bloated build systems!