BUG: allow insertion/deletion of columns in non-unique column DataFrames
closes #3679, #3687

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
```

Don't try this at home
1) duplicates across dtypes
2) assigning those duplicates

```
In [5]: df = DataFrame([[1,1,1.,5],[1,1,2.,5],[2,1,3.,5]],columns=['foo','bar','foo','hello'])

In [6]: df.dtypes
Out[6]: 
foo        int64
bar        int64
foo      float64
hello      int64
dtype: object

In [7]: df['foo'] = 'string'

In [8]: df
Out[8]: 
      foo  bar     foo  hello
0  string    1  string      5
1  string    1  string      5
2  string    1  string      5
```

"A bit non trivial" lol

Is this still WIP? I think the example from above is still present (if I my HEAD is on this correctly)...
 this works on this branch (not on dev)
 Weird, #3687 is still not working for me on the branch, tip eb3720f (maybe I'm doing it wrong!).
 something funny going on....works in this case:
but not in the other.....will fix..thanks

```
(Pdb) idx = date_range('20130101',periods=4,freq='Q-NOV')
(Pdb) df = DataFrame([[1,1,1,5],[1,1,2,5],[2,1,3,5]],columns=['a','a','a','a'])
(Pdb) df.columns = idx
(Pdb) df
   2013-02-28  2013-05-31  2013-08-31  2013-11-30
0           1           1           1           5
1           1           1           2           5
2           2           1           3           5
```
 was an oversite in the constructor, was constructing the _ref_locs when mixed type of in the example I used...., but not in a single-dtype array....will be updated soon
 @wesm a bit more complicated that I like
but necessary - pls give a once over
