API: deprecate get_offset, Offset for time rules like '30s'
From stackoverflow: http://stackoverflow.com/questions/17579932/statsmodel-arma-function-is-incompatiable-with-pandas/17581170

I'm looking only quickly at this, but I'm wondering if this should be recognized and return an offset.

```
from pandas.tseries.frequencies import get_offset
get_offset('30s')
```

or if in statsmodels we should accept custom offsets in place of frequency strings.
