Renamed Scala SimpleResult to Result

This is a follow up to the results refactoring done for Play 2.2.  It was intended that Play 2.3 removed the deprecated types and renamed SimpleResult to Result.

This is being done in multiple steps, each step being a separate commit, to ensure that the change is source compatible while also ensuring that our code and documentation is fully updated.  When reviewing, it may be worth reviewing these commits one at a time.

### Step 1 - The minimal change

The idea of this step was to change the minimum things possible, essentially:

* Renamed SimpleResult to Result
* Removed all deprecated result types
* Merged WithHeaders into Result since it only existed for the purpose of sharing functionality between PlainResult AsyncResult

Then I introduced a `SimpleResult` type alias and object, marking them as deprecated.  Then I changed the minimum of the rest of the code to get it to compile.  Nearly everything that had to be changed was just Java helper source files, since these source files don't recognise the type alias.  But they were only internal helpers for Play, nothing exposed to users.  All sample code/documentation/public APIs remained unchanged.

This proved that the changes are source compatible, and so won't be too disruptive for users.  I submitted this commit as the initial PR, and Jenkins passed it.

### Step 2 - Comprehensive update

In this step I commented out `SimpleResult`, which made everything fail to to compile.  I then updated all the Play core code, tests, docs, samples etc to use the new `Result` type.  This proved that all of our code and docs has been updated to use the new `Result` type.

There are of course many instances where we refer to `SimpleResult` in comments and documentation that doesn't get verified, I used grep and other tools to do my best here.

I added this commit to the PR, and Jenkins passed it.

### Step 3 - Reintroduce `SimpleResult` type alias

In this step I reintroduced the `SimpleResult` type alias and object, so that end users can use it.  I also added instructions to migrate to the new type in the 2.3 migration guide.