BUG: allow insertion/deletion of columns in non-unique column DataFrames

closes #3679

Here's example of various operations

```
In [3]: df = DataFrame([[1,1,1,5],[1,1,2,5],[2,1,3,5]],
             columns=['foo','bar','foo','hello'])

In [27]: df.columns.is_unique
Out[27]: False

In [4]: # insert

In [5]: df['string'] = 'bah'

In [6]: df
Out[6]: 
   foo  bar  foo  hello string
0    1    1    1      5    bah
1    1    1    2      5    bah
2    2    1    3      5    bah

In [7]: # insert same dtype

In [8]: df['foo2'] = 3

In [9]: df
Out[9]: 
   foo  bar  foo  hello string  foo2
0    1    1    1      5    bah     3
1    1    1    2      5    bah     3
2    2    1    3      5    bah     3

In [10]: # delete (non dup)

In [11]: del df['bar']

In [12]: df
Out[12]: 
   foo  foo  hello string  foo2
0    1    1      5    bah     3
1    1    2      5    bah     3
2    2    3      5    bah     3

In [13]: # try to delete again (its not consolidated)

In [14]: del df['hello']

In [15]: df
Out[15]: 
   foo  foo string  foo2
0    1    1    bah     3
1    1    2    bah     3
2    2    3    bah     3

In [16]: # insert

In [17]: df.insert(2,'new_col',5.)

In [18]: df
Out[18]: 
   foo  foo  new_col string  foo2
0    1    1        5    bah     3
1    1    2        5    bah     3
2    2    3        5    bah     3
```

This is the current default behavior now
```
In [19]: # insert a dup

In [20]: df.insert(2,'new_col',4.)
Exception: cannot insert new_col, already exists

In [21]: # insert a dup

In [22]: df.insert(2,'new_col',4.,allow_duplicates=True)

In [23]: df
Out[23]: 
   foo  foo  new_col  new_col string  foo2
0    1    1        4        5    bah     3
1    1    2        4        5    bah     3
2    2    3        4        5    bah     3
```

```
In [24]: # delete (dup)

In [25]: del df['foo']

In [26]: df
Out[26]: 
   new_col  new_col string  foo2
0        4        5    bah     3
1        4        5    bah     3
2        4        5    bah     3
```