[DOC] Replace invalid emojis

#### What does this MR do?

This PR replaces invalid emojis. Additionally it links to the emoji cheatsheet on [dropbox](https://www.dropbox.com/s/b9xaqb977s6d8w1/cheat_sheet.pdf) @dosire Is that ok? Point to the original cheatsheet doesn't make sense, because the symbols are quite different.

#### Why was this MR needed?

With the switch to the Phantom Open emoji library, not all emojis are valid anymore and aren't shown properly anymore.

#### Screenshots 
##### Before
![screenshot 2014-08-03 11 50 37](https://cloud.githubusercontent.com/assets/278301/3790248/ab774332-1af3-11e4-9782-8cd9bc051160.png)

##### After

![screenshot 2014-08-03 11 51 35](https://cloud.githubusercontent.com/assets/278301/3790252/c8d18bfe-1af3-11e4-85ff-1a9487bb838c.png)