Implement summarize statistics function to Frame similar to describe() in pandas

Would be grate to have function which returns summary of dataframe like in pandas describe() function

```
import datatable as dt
import pandas as pd

DT1 = dt.Frame(product_id=[1,2,3], price=[1,2,3])
df1 = DT1.to_pandas()

print(df1)
print(df1.describe())
```
returns:

```
product_id price
0 1 1
1 2 2
2 3 3
product_id price
count 3.0 3.0
mean 2.0 2.0
std 1.0 1.0
min 1.0 1.0
25% 1.5 1.5
50% 2.0 2.0
75% 2.5 2.5
max 3.0 3.0
```