Add capability to specify separator to to_csv

Just a note: Before adding this feature, there were a couple of existing tests failing. I added a test for this feature though, and the output from rake test is the same as before the change. 

See below for output from rake test (running ruby 1.9.3p392), if this is not expected and existing tests are actually not breaking for you.

```
  1) Failure:
test_bug_datetime_to_csv(TestRoo) [/Users/nicke/code/roo/test/test_roo.rb:1887]:
<""> expected but was
<"<1961-11-21T12:17:18+00:00,1961-11-21T12:17:18+00:00,1961-11-21T12:17:18+00:00\n>1961-11-21T12:17:17+00:00,1961-11-21T12:17:17+00:00,1961-11-21T12:17:17+00:00\n<1961-11-21T12:17:18+00:00,1961-11-21T12:17:18+00:00,1961-11-21T12:17:18+00:00\n>1961-11-21T12:17:17+00:00,1961-11-21T12:17:17+00:00,1961-11-21T12:17:17+00:00\n<1961-11-21T12:17:18+00:00,1961-11-21T12:17:18+00:00,1961-11-21T12:17:18+00:00\n>1961-11-21T12:17:17+00:00,1961-11-21T12:17:17+00:00,1961-11-21T12:17:17+00:00\n">.

  2) Error:
test_bug_pfand_from_windows_phone_xlsx(TestRoo):
Encoding::CompatibilityError: incompatible encoding regexp match (US-ASCII regexp with UTF-16 string) for excelx
(... removed stacktrace ...)

  3) Failure:
test_datetime(TestRoo) [/Users/nicke/code/roo/test/test_roo.rb:1369]:
<#<DateTime: 1961-11-21T12:17:18+00:00 ((2437625j,44238s,0n),+0s,2299161j)>> expected but was
<#<DateTime: 1961-11-21T12:17:17+00:00 ((2437625j,44237s,0n),+0s,2299161j)>>.
```