delimiter in CSV writer should support unicode

I think the following should probably be made to work:

```
In [6]: t.write('test.csv', format='ascii', delimiter=u',')
ERROR: TypeError: "delimiter" must be an 1-character string [astropy.io.ascii.core]
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-6-1f963bd3edbd> in <module>()
----> 1 t.write('test.csv', format='ascii', delimiter=u',')

/Users/tom/Library/Python/2.7/lib/python/site-packages/astropy-0.4.dev6923-py2.7-macosx-10.8-x86_64.egg/astropy/table/table.pyc in write(self, *args, **kwargs)
   1732         passed through to the underlying data reader (e.g. `~astropy.io.ascii.ui.write`).
   1733         """
-> 1734         io_registry.write(self, *args, **kwargs)
   1735 
   1736     def copy(self, copy_data=True):

/Users/tom/Library/Python/2.7/lib/python/site-packages/astropy-0.4.dev6923-py2.7-macosx-10.8-x86_64.egg/astropy/io/registry.pyc in write(data, *args, **kwargs)
    356 
    357     writer = get_writer(format, data.__class__)
--> 358     writer(data, *args, **kwargs)
    359 
    360 

/Users/tom/Library/Python/2.7/lib/python/site-packages/astropy-0.4.dev6923-py2.7-macosx-10.8-x86_64.egg/astropy/io/ascii/connect.pyc in write_asciitable(table, filename, **kwargs)
     23 def write_asciitable(table, filename, **kwargs):
     24     from .ui import write
---> 25     return write(table, filename, **kwargs)
     26 
     27 io_registry.register_writer('ascii', Table, write_asciitable)

/Users/tom/Library/Python/2.7/lib/python/site-packages/astropy-0.4.dev6923-py2.7-macosx-10.8-x86_64.egg/astropy/io/ascii/ui.pyc in write(table, output, format, Writer, **kwargs)
    293     Writer = _get_format_class(format, Writer, 'Writer')
    294     writer = get_writer(Writer=Writer, **kwargs)
--> 295     lines = writer.write(table)
    296 
    297     # Write the lines to output

/Users/tom/Library/Python/2.7/lib/python/site-packages/astropy-0.4.dev6923-py2.7-macosx-10.8-x86_64.egg/astropy/io/ascii/core.pyc in write(self, table)
    914         # Write header and data to lines list
    915         lines = []
--> 916         self.header.write(lines)
    917         self.data.write(lines)
    918 

/Users/tom/Library/Python/2.7/lib/python/site-packages/astropy-0.4.dev6923-py2.7-macosx-10.8-x86_64.egg/astropy/io/ascii/core.pyc in write(self, lines)
    414                                        itertools.cycle(self.write_spacer_lines)):
    415                 lines.append(spacer_line)
--> 416             lines.append(self.splitter.join([x.name for x in self.cols]))
    417 
    418     @property

/Users/tom/Library/Python/2.7/lib/python/site-packages/astropy-0.4.dev6923-py2.7-macosx-10.8-x86_64.egg/astropy/io/ascii/core.pyc in join(self, vals)
    294                                          quotechar=self.quotechar,
    295                                          quoting=self.quoting,
--> 296                                          lineterminator='',
    297                                          )
    298         self.csv_writer_out.seek(0)

TypeError: "delimiter" must be an 1-character string
```

I found this in a script that was importig ``unicode_literals`` from ``__future__``

cc @taldcroft