PERF: optimize index.__getitem__ for slice & boolean mask indexers

This patch fixes performance issues discussed in #6370.

There's an API change though: no type inference will now happen upon resulting index, e.g. here's what it would look like on current master:
```python
In [1]: pd.Index([1,2,3, 'a', 'b', 'c'])
Out[1]: Index([1, 2, 3, u'a', u'b', u'c'], dtype='object')

In [2]: _1[[0,1,2]]
Out[2]: Int64Index([1, 2, 3], dtype='int64')
```

And this is how it looks with the patch:
```python
In [1]: pd.Index([1,2,3, 'a', 'b', 'c'])
Out[1]: Index([1, 2, 3, u'a', u'b', u'c'], dtype='object')

In [2]: _1[[0,1,2]]
Out[2]: Index([1, 2, 3], dtype='object')
```

On the bright side, I'd say using mixed-type indices seems a quite rare scenario and the performance improvement is worth it.

## Performance Before:

```python
In [1]: import pandas.util.testing as tm

In [2]: idx = tm.makeStringIndex(1000000)

In [3]: mask = np.arange(1000000) % 3 == 0

In [4]: series_mask = pd.Series(mask); mask
Out[4]: array([ True, False, False, ..., False, False,  True], dtype=bool)

In [5]: timeit idx[:-1]
100000 loops, best of 3: 1.57 µs per loop

In [6]: timeit idx[::2]
100 loops, best of 3: 14.6 ms per loop

In [7]: timeit idx[mask]
100 loops, best of 3: 15.4 ms per loop

In [8]: timeit idx[series_mask]
100 loops, best of 3: 15.4 ms per loop

In [9]: pd.__version__
Out[9]: '0.13.1-278-gaf63b99'
```

## Performance After:
```python
In [1]: import pandas.util.testing as tm

In [2]: idx = tm.makeStringIndex(1000000)

In [3]: mask = np.arange(1000000) % 3 == 0

In [4]: series_mask = pd.Series(mask)

In [5]: timeit idx[:-1]
1000000 loops, best of 3: 1.56 µs per loop

In [6]: timeit idx[::2]
100000 loops, best of 3: 4.89 µs per loop

In [7]: timeit idx[mask]
100 loops, best of 3: 11.4 ms per loop

In [8]: timeit idx[series_mask]
100 loops, best of 3: 11.4 ms per loop

In [9]: pd.__version__
Out[9]: '0.13.1-279-gbc810f0'

```