pivot_table raises DataError: No numeric types to aggregate with Timedelta
#### Code Sample, a copy-pastable example if possible

```python
In [1]: import pandas as pd

In [2]: df = pd.DataFrame({"Name": ["A", "B", "C", "B", "A", "C"], "Laptime": [48.0,48.0,48.0,48.0,48.0,48.0], "Time": [12.0, 14, 16, 62, 60, 64]})

In [3]: df.dtypes
Out[3]:
Name        object
Laptime    float64
Time       float64
dtype: object

In [4]: pd.pivot_table(df, values="Laptime", index="Time", columns="Name")
```

returns 

```
Name     A     B     C
Time
12.0  48.0   NaN   NaN
14.0   NaN  48.0   NaN
16.0   NaN   NaN  48.0
60.0  48.0   NaN   NaN
62.0   NaN  48.0   NaN
64.0   NaN   NaN  48.0
```

but when columns and/or index are `Timedelta`, `pivot_table` raises an exception

```
In [5]: df = pd.DataFrame({"Name": ["A", "B", "C", "B", "A", "C"], "Laptime": [48.0,48.0,48.0,48.0,48.0,48.0], "Time": [12.0, 14, 16, 62, 60, 64]})
In [6]: for colname in ["Laptime", "Time"]:
            df[colname] = pd.to_timedelta(df[colname], unit='s')
In [7]: df.dtypes
Out[7]:
Name                object
Laptime    timedelta64[ns]
Time       timedelta64[ns]
dtype: object
In [8]: pd.pivot_table(df, values="Laptime", index="Time", columns="Name")
```

it raises 

    DataError: No numeric types to aggregate

#### Problem description

raises:

    DataError: No numeric types to aggregate

#### Expected Output

shouldn't raises exception but should return


```
Name             A         B         C
Time
00:00:12  00:00:48       NaT       NaT
00:00:14       NaT  00:00:48       NaT
00:00:16       NaT       NaT  00:00:48
00:00:60  00:00:48       NaT       NaT
00:00:62       NaT  00:00:48       NaT
00:00:64       NaT       NaT  00:00:48
```

#### Output of ``pd.show_versions()``

<details>

pd.show_versions()
/Users/scls/anaconda3/lib/python3.7/site-packages/psycopg2/__init__.py:144: UserWarning: The psycopg2 wheel package will be renamed from release 2.8; in order to keep installing from binary please use "pip install psycopg2-binary" instead. For details see: <http://initd.org/psycopg/docs/install.html#binary-install-from-pypi>.
  """)

INSTALLED VERSIONS
------------------
commit: None
python: 3.7.3.final.0
python-bits: 64
OS: Darwin
OS-release: 17.7.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: fr_FR.UTF-8
LOCALE: fr_FR.UTF-8

pandas: 0.24.2
pytest: 4.5.0
pip: 19.1
setuptools: 41.0.1
Cython: 0.29.7
numpy: 1.16.4
scipy: 1.2.1
pyarrow: None
xarray: None
IPython: 7.5.0
sphinx: 2.0.1
patsy: 0.5.1
dateutil: 2.8.0
pytz: 2019.1
blosc: None
bottleneck: 1.2.1
tables: 3.5.1
numexpr: 2.6.9
feather: None
matplotlib: 3.0.3
openpyxl: 2.6.1
xlrd: 1.2.0
xlwt: 1.3.0
xlsxwriter: 1.1.8
lxml.etree: 4.3.3
bs4: 4.7.1
html5lib: 1.0.1
sqlalchemy: 1.3.3
pymysql: 0.9.3
psycopg2: 2.7.7 (dt dec pq3 ext lo64)
jinja2: 2.10.1
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
gcsfs: None

</details>
