gr_uhd: added subtraction and addition to time_spec_t

Fixing a long lasting problem:
```python
from gnuradio import uhd
t = uhd.time_spec_t(42.0)
sum = t + t
### Operator not defined.
```

This is due to SWIG not understanding Boost's operator.hpp automatic operator generation magic.

Fixed by extending the time_spec_t class from withing SWIG.

Minimal self test passes.