Offset updates

Update offset() parameter handling as discussed in #483.

I'll update the test cases if we agree to go on with that behavior...

- Rounded offset is now using parameter r
- Square offset uses delta = x and bevel_limit = 0
- Miter offset uses delta = x and bevel_limit >= 2
- Default bevel_limit, if not given is 2
- If bevel_limit is < 2 and <> 0, a warning is given and Square is used

![openscad-offset](https://cloud.githubusercontent.com/assets/1330241/4965140/207a0eee-677f-11e4-844e-efad57dc2cf6.png)