Replaced hard-coded instances of "adc5g_controller" with OPB_CONTROLLER.
Added a function for reading off histogram data.

To use it do the following:

```python
adc5g.get_histogram(roach, zdok_n, core)
```

Where core is one of ['a', 'b'. 'c', 'd']. This will return a list of the
histogram from the last integration. This data will be ordered from -size/2
to +size/2-1. By default size=256.