Sentinal -> Sentinel
I was working on some typo fixes, and noticed that "Sentinel" is misspelled "Sentinal" in a few user-visible places (function names, arguments).  Seems unlikely that anyone's relying on this, though, and it would produce a noisy and easily fixed error if so after the change.

```
./pandas/core/common.py:def sentinal_factory():
./pandas/core/common.py:    class Sentinal(object):
./pandas/core/common.py:    return Sentinal()
./pandas/core/format.py:            sentinal = com.sentinal_factory()
./pandas/core/format.py:            levels = self.columns.format(sparsify=sentinal, adjoin=False,
./pandas/core/format.py:            level_lengths = _get_level_lengths(levels, sentinal)
./pandas/core/format.py:            sentinal = com.sentinal_factory()
./pandas/core/format.py:            levels = frame.index[:nrows].format(sparsify=sentinal,
./pandas/core/format.py:            level_lengths = _get_level_lengths(levels, sentinal)
./pandas/core/format.py:def _get_level_lengths(levels, sentinal=''):
./pandas/core/format.py:            if x != sentinal:
./pandas/core/index.py:            sentinal = ''
./pandas/core/index.py:            # use value of sparsify as sentinal,  unless it's an obvious
./pandas/core/index.py:                sentinal = sparsify
./pandas/core/index.py:                                      sentinal=sentinal)
./pandas/core/index.py:def _sparsify(label_list, start=0, sentinal=''):
./pandas/core/index.py:                sparse_cur.append(sentinal)
./doc/source/missing_data.rst:sentinal value that can be represented by numpy in a singular dtype (datetime64[ns]).
```
