Added Line length section. The provided examples all meet this length.

As I mentioned in the issue [#53](https://github.com/raywenderlich/objective-c-style-guide/issues/53) it is not mentioned in this guide the recommended line length. So I added this small section, setting it to 100. In my experience this is good enough and so recommends Google [style guide](https://google-styleguide.googlecode.com/svn/trunk/objcguide.xml#Line_Length). All the provided examples in this guide meet this criteria. I also mentioned how this column guide can be set in Xcode.