BUG in clipboard (linux, python2) with unicode and separator
This is probably a known bug but I couldn't find a github issue.

There is a disabled test `test_clipboard.py` which fails with the following error

``` python
======================================================================
FAIL: test_round_trip_frame_sep (pandas.io.tests.test_clipboard.TestClipboard)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/users/piotr/workspace/pandas-pijucha/pandas/io/tests/test_clipboard.py", line 73, in test_round_trip_frame_sep
    self.check_round_trip_frame(dt, sep=',')
  File "/home/users/piotr/workspace/pandas-pijucha/pandas/io/tests/test_clipboard.py", line 69, in check_round_trip_frame
    tm.assert_frame_equal(data, result, check_dtype=False)
  File "/home/users/piotr/workspace/pandas-pijucha/pandas/util/testing.py", line 1276, in assert_frame_equal
    right.columns))
  File "/home/users/piotr/workspace/pandas-pijucha/pandas/util/testing.py", line 1022, in raise_assert_detail
    raise AssertionError(msg)
AssertionError: DataFrame are different

DataFrame shape (number of columns) are different
[left]:  2, Index([u'en', u'es'], dtype='object')
[right]: 0, Index([], dtype='object')
```
#### Code Sample, a copy-pastable example if possible

More explicitly (the example from the above test):

``` python
nonascii = pd.DataFrame({'en': 'in English'.split(), 'es': 'en español'.split()})

nonascii.to_clipboard(sep=',')

read_clipboard(sep=',', index_col=0)
Out[154]: 
Empty DataFrame
Columns: []
Index: [0       in       en, 1  English  español]

read_clipboard()
Out[155]: 
        en       es
0       in       en
1  English  español
```
#### Expected Output

``` python
read_clipboard(sep=',', index_col=0)
Out[134]: 
        en       es
0       in       en
1  English  español

read_clipboard()
Out[135]: 
              ,en,es
0            0,in,en
1  1,English,español
```
#### output of `pd.show_versions()`

```
INSTALLED VERSIONS
------------------
commit: None
python: 2.7.11.final.0
python-bits: 64
OS: Linux
OS-release: 4.1.20-1
machine: x86_64
processor: Intel(R)_Core(TM)_i5-2520M_CPU_@_2.50GHz
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.18.1+240.gbb6b5e5
nose: 1.3.7
pip: 8.1.2
setuptools: 21.2.0
Cython: 0.24.1
numpy: 1.11.0
```

---

There are probably 2 issues in the code.
1. `.encode('utf-8') [is called](https://github.com/pydata/pandas/blob/bb6b5e54edaf046389e8cce28e7cd27ee87f5fcc/pandas/util/clipboard.py#L172) on a py2 string, which raises if there is a non-ascii character in the string, and then
2. `to_clipboard` [falls back](https://github.com/pydata/pandas/blob/bb6b5e54edaf046389e8cce28e7cd27ee87f5fcc/pandas/io/clipboard.py#L87) to `to_string` method.
   (In this case, fixing 1 solves the problem. But in general, if something else raises and we fall back here, a separator is ignored.)

I don't know what to do about 2, but 1 seems to be easy. 
Part of the code in `util.clipboard.py` calls `subprocess.Popen.communicate()`, which operates on byte types (bytes in PY3 and strings in PY2). So, `encode`/`decode` are needed only in PY3.

I believe this https://github.com/pydata/pandas/commit/6d4fdb0e9e0c55caacd60db8989f6868bf6fea0a  fixes the problem. But for now I tested only one pair of functions (in KDE) and couldn't possibly test it on OS X.
