Add support for multi-row headings

Horizontal space is limited in consoles and I needed several groups of related columns. In order to save space, I added support for multi-row headings and used this in conjunction with colspan. It works very well.

```
+------+------+
| Char | Num  |
+------+------+
| 2nd heading |
+------+------+
| a    | 1    |
| b    | 2    |
| c    | 3    |
+------+------+
```

I also had some trouble running the specs so I fixed that as well as a problem with the gemspec.