BUG: Various issues with maybe_convert_objects
Found some issues with `lib.maybe_convert_objects` (which is an internal library function, but the same problems as below occur when calling `DataFrame({'a': [...]})`, for example)

(Also, mostly unrelated to the below, I noticed the inference loop continues for no reason after seeing an object, when it can safely break because the output is a forgone conclusion: added the breaks to make it slightly more efficient.)
##### Fails on python long integer outside `np.int64` range:

``` python
In [4]: arr = np.asarray([2**63], dtype=np.object_)

In [5]: p.lib.maybe_convert_objects(arr)
(exception)
```

Probably should leave the integer unchanged as a long integer object
##### Datetimes and complexes lost completely when mixed with booleans:

It's not even doing a cast: it's reading uninitialized memory as boolean

``` python
In [6]: arr = np.asarray([dt.datetime(2005, 1, 1), True])

In [7]: p.lib.maybe_convert_objects(arr, convert_datetime=1)
Out[7]: array([ True,  True], dtype=bool)

In [8]: arr = np.asarray([1.0+2.0j, True], dtype=np.object_)

In [9]: p.lib.maybe_convert_objects(arr)
Out[9]: array([ True,  True], dtype=bool)
```
##### `None` converts to `np.nan` with floats, but not complexes:

``` python
In [10]: arr = np.asarray([1.0, None], dtype=np.object_)

In [11]: p.lib.maybe_convert_objects(arr)
Out[11]: array([  1.,  nan])

In [12]: arr = np.asarray([1.0+2.0j, None], dtype=np.object_)

In [13]: p.lib.maybe_convert_objects(arr)
Out[13]: array([(1+2j), None], dtype=object)
```
##### `safe` option preserves integer types with `np.nan` but not `None`:

The comment for `safe` says "don't cast int to float, etc." but that is not true with `None`

``` python
In [14]: arr = np.asarray([1, 2.0, np.nan], dtype=np.object_)

In [15]: p.lib.maybe_convert_objects(arr, safe=1)
Out[15]: array([1, 2.0, nan], dtype=object)

In [16]: arr = np.asarray([1, 2.0, None], dtype=np.object_)

In [17]: p.lib.maybe_convert_objects(arr, safe=1)
Out[17]: array([  1.,   2.,  nan])
```

Not entirely sure if this last one is intended behavior or not; let me know if there is any feedback.
