index into multi-index past the lex-sort depth
closes https://github.com/pydata/pandas/issues/7724
closes https://github.com/pydata/pandas/issues/2646

on current master:

```
>>> df
                           jolia
jim joe jolie      joline       
1   z   2014-10-14 a          30
    y   2014-10-13 b           3
    x   2014-10-12 c          15
0   z   2014-10-11 d          35
    y   2014-10-10 e          43
    x   2014-10-09 f          36
```

False negative if the key length exceeds lexsort depth:

```
>>> (0,) in df.index
False
>>> (0, 'z') in df.index
False
>>> (0, 'z', '2014-10-11') in df.index
False
>>> (0, 'z', Timestamp('2014-10-11')) in df.index
False
>>> (0, 'z', '2014-10-11', 'd') in df.index
False
```

only ones which work:

```
>>> 0 in df.index
True
>>> (0, 'z', Timestamp('2014-10-11'), 'd') in df.index
True
```

which take a different code paths. The last one only works if the index is unique:

```
>>> (0, 'z', Timestamp('2014-10-11'), 'd') in pd.concat([df, df]).index
False
```

for all of the false negative cases above, obviously `df.loc[key]` fails:

```
>>> df.loc[(0, 'z', Timestamp('2014-10-11'))]
KeyError: 'Key length (3) was greater than MultiIndex lexsort depth (0)'
```

_Some of these issues persist even if the index is lexically sorted:_

```
>>> df.sort_index(inplace=True)
>>> df  # lexically sorted
                           jolia
jim joe jolie      joline       
0   x   2014-10-09 f          36
    y   2014-10-10 e          43
    z   2014-10-11 d          35
1   x   2014-10-12 c          15
    y   2014-10-13 b           3
    z   2014-10-14 a          30
```

date-time indexing with a full-key fails if index is unique:

```
>>> (0, 'x', '2014-10-09') in df.index  # partial key, works!
True
>>> (0, 'x', '2014-10-09', 'f') in df.index  # full key, unique index, breaks!
False
```

also, non-unique lexically sorted index always returns false positive with any full key:

```
>>> xdf = pd.concat([df, df]).sort_index()
>>> xdf
                           jolia
jim joe jolie      joline       
0   x   2014-10-09 f          36
                   f          36
    y   2014-10-10 e          43
                   e          43
    z   2014-10-11 d          35
                   d          35
1   x   2014-10-12 c          15
                   c          15
    y   2014-10-13 b           3
                   b           3
    z   2014-10-14 a          30
                   a          30
>>> (0, '$', '2014-10-09') in xdf.index  # partial key works
False
>>> (0, '$', '2014-10-09', '#') in xdf.index  # full key always `True`
True
>>> xdf.loc[(0, '$', '2014-10-09', '#')]
KeyError: 'the label [$] is not in the [columns]'
```
