Advanced logging system

- You can now define a custom `LogStyle`
- Added `warn()`, `error()`, `notice()` and `advancedLog()` to `FlxG`
- `warn()`, `error()`, `notice()` and `log()` support a variable amount of
paramaters. This means that to get an output of `"X: 500 Y: 250"`, you no
longer need to seperate everything with plus signs (`log("X: " + this.x +
" Y: " + this.y);`), instead you can just have seperate paramters:
`log("X: ", this.x, "Y:", this.y);` which is more convenient and readable.
- Updated all the existing error and warning messages previously using
`log()`

Note that the console code hasn't been updated, even though it uses a ton of logging. I will do that later when the `dev` and `texture_atlas` branches are merged, because that's where the most recent version of the console code is.

#361

Huge thanks to the [ideas over at flixel-community](https://github.com/FlixelCommunity/flixel/issues/89) regarding the enhancement of the logging system. 

I think two more sound effects should be included in core flixel, one to be used in conjunction with the `warn()` and one in conjunction with the `error()` log. Currently, they both use the `"Beep"` soundeffect, which works, but is not very appropriate.