ENH: summarize over observations

Fixes #104
@wasade and I talked offline, this now makes sense to me.  The code is just for displaying the observation metadata keys, not the values (so an example output looks like this, if **taxonomy** is the only observation metadata in the table):
```
Num samples: 384
Num observations: 1134
Total count: 6771251
Table density (fraction of non-zero values): 0.089

Counts/sample summary:
 Min: 1.0
 Max: 4846228.0
 Median: 9.000
 Mean: 5971.121
 Std. dev.: 144739.758
 Sample Metadata Categories: None provided
 Observation Metadata Categories: taxonomy

Counts/sample detail:
 2359343: 1.0
 4479288: 1.0
 818063: 1.0
[truncated]
```
So I am fine with this the way it is!  :+1: