API: initially commit for error handling

~~**NOT READY FOR MERGE**~~

This PR contains the initial error handling API that is fairly similar to numpy's system, and includes a context manager. Currently, only empty table is defined, but adding other error types is easy. In addition, (and I didn't know numpy had this until today), you can define callbacks for errors. I have not incorporated this yet into `Table`, and of course more tests and examples are necessary to add. 

I wanted to get this in now to check if this seems like a viable solution to handling empty tables and other error types. @gregcaporaso @jairideout, I wanted to flag you on this too as this may be more generally useful for use in scikit-bio, though it may be useful to define multiple profiles (e.g., one for `DistanceMatrix`, `TreeNode`, etc). 

Examples of what should be possible with this code once adopted by `Table`:

```python
>>> from biom import example_table
>>> from biom.err import seterr, seterrcall, errstate 
>>> seterr(emptytable='ignore')
>>> # filter to an empty table
>>> t = example_table.filter(lambda v, i, md: v.sum() > 100000000, inplace=False)
>>> # raise if we create an empty table
>>> with errstate(emptytable='raise'):
...    t = example_table.filter(lambda v, i, md: v.sum() > 100000000, inplace=False)
Traceback (most recent call last):
...
biom.exception.TableException: All data was filtered out!
>>> # our original 'ignore' state is back to being valid
>>> t = example_table.filter(lambda v, i, md: v.sum() > 100000000, inplace=False)
>>> # show example of a callback
>>> def callback():
...    print "damnit"
>>> seterrcall('emptytable', callback)   # may want this to be kwarg like seterr
>>> seterr(emptytable='call')
>>> t = example_table.filter(lambda v, i, md: v.sum() > 100000000, inplace=False)
damnit
```
Gotcha -- I missed where the lambda functions were being executed (in ```_handle_error```), so this makes sense, and allows greater flexibility in the case that people want to extend this class. :+1: