frequency issues
I'm just trying to get my head around what these tools actually do and what they're used for. I can't tell if I'm misunderstanding or this is all just unfinished.

```
from pandas.tseries import frequencies
frequencies._is_annual('AS') # oversight?
# can I go from quarterly to annual?
frequencies.is_subsample('Q', 'A') # yes
frequencies.is_subsample('Q', 'AS') # none?
frequencies.is_subperiod(frequencies.to_offset('Q'), frequencies.to_offset('AS')) # none
# what about weekly to annual?
frequencies.is_subperiod('W', 'A') # false?
```

 It looks like a combination of _is_annual and friends not also checking for `AS-`, etc. and the omission of frequency strings in the checks in `is_subperiod`. Easy fix, just want some sanity checking.
