Option to prefix with slashes

I have always used three slashes as a prefix for appledoc documentation and thought it might be a useful option to add to this project. The pull request adds a new `prefixWithSlashes` option. The settings screen now shows a matrix of radio buttons allowing the user to choose between the 3 prefix options. The slashes option is disabled when HeaderDoc comments are enabled, since it is only supported by Appledoc.

![New prefix options](https://f.cloud.github.com/assets/507058/1297343/ed8fe9b2-30df-11e3-9893-fea6793e352e.png)

One caveat to using `///` for comments is that the `///` keyword causes issues when adding new lines to the comment. I would try to add a second line to the description but the plugin picked up the `///` trigger and inserted a second copy of the documentation block. I had to add some logic to avoid generating the comment if the previous or next lines in the editor are `///` documentation comments.

I hope this is useful, please let me know if anything needs to be fixed before the change can be merged.

### Why `///` instead of stars or whitespace?

If anyone is wondering why I use `///`, the reason is fairly simple, it's more reliable in Appledoc. Consider the following documentation that contains example code. In Appledoc, example code is simply indented 4 or more spaces.
```objc
/// This method does something very useful.
///
/// The following example shows an appropriate use of `doSomething`
///
///    NSLog(@"%@", thing.someProperty);  // Logs "A"
///    [thing doSomething];
///    NSLog(@"%@", thing.someProperty);  // Logs "B"
- (void)doSomething;
```
This generates the following in Appledoc, looks pretty good.
> ![Documentation example with 3 slashes](https://f.cloud.github.com/assets/507058/1297526/3e349508-30e4-11e3-9fa2-d5f973253219.png)

-----

However, if we try the same code with star-prefixed comments the result is much different. This is probably a bug in Appledoc, but it has been around for a long time. Examples are simply not picked up when a star prefix is used.
```objc
/**
 * This method does something very useful.
 *
 * The following example shows an appropriate use of `doSomething`
 * 
 *     NSLog(@"%@", thing.someProperty);  // Logs "A"
 *     [thing doSomething];
 *     NSLog(@"%@", thing.someProperty);  // Logs "B"
 */
- (void)doSomething;
```
> ![Documentation example with star prefix](https://f.cloud.github.com/assets/507058/1297531/869c8e2c-30e4-11e3-948a-111c3fd79b12.png)

-----

It may seem to work fine without the star prefix.
```objc
/**
  This method does something very useful.

  The following example shows an appropriate use of `doSomething`

      NSLog(@"%@", thing.someProperty);  // Logs "A"
      [thing doSomething];
      NSLog(@"%@", thing.someProperty);  // Logs "B"
 */
```
> ![Documentation with whitespace prefix](https://f.cloud.github.com/assets/507058/1297551/e515f114-30e4-11e3-8496-600effe73811.png)

-----

However, Appledoc does not have a concept of a base indentation level within a comment. If any line in this comment format is 4 or more spaces from the left it will be rendered as an example block. Yikes!
```objc
/**
    This method does something very useful.

    The following example shows an appropriate use of `doSomething`

        NSLog(@"%@", thing.someProperty);  // Logs "A"
        [thing doSomething];
        NSLog(@"%@", thing.someProperty);  // Logs "B"
 */
- (void)doSomething;
```
> ![Documentation with indented whitespace prefix](https://f.cloud.github.com/assets/507058/1297585/3896d754-30e5-11e3-9ec4-770f4fc9cd47.png)

Finally, **this is very important**, if you have been relying on indentation to offset example code in Appledoc comments without a `///` or `*` prefix, know that XCode has no interest in preserving your indentation. If you ever try to copy and paste those comments or use the *Reindent* option on your code, Xcode will align everything in the comment to the same column as the first star in the comment opening. Combined with the fact that the star prefix does not parse properly in Appledoc, this is why I use `///` as a prefix.