Rewrote content builder targets

### What it does

1. Automatically builds MonoGame Content projects before the project is built
2. Builds incrementally to allow faster development
3. Supports multiple projects in the same directory (like resolution packs), whitespaces in paths (like the Visual Studio 2013 folder)
4. Outputs built content to `ProjectDirectory\Content\PlatformName` (avoiding clash of contents built for different platforms)
5. Content files will be added to the project and copied to the output directory
6. Content files will be automatically packaged with the rest of the app (including Android)

TL;DL
Works as seamlessly as XNA content pipeline

### How to get it to work
Add the following lines to the csproj file using a text editor:
#### At the end of the first `<PropertyGroup>`:
```XML
<MonoGamePlatform>PlatformName</MonoGamePlatform>
```
(PlatformName shuold be iOS, Android, WindowsPhone, etc)

#### Somewhere after an `</ItemGroup>`:
```XML
  <ItemGroup>
    <MonoGameContentReference Include="PathToYour.mgcb">
    </MonoGameContentReference>
  </ItemGroup>
```

#### At the end of the file, before `</Project>`
```XML
  <Import Project="PathTo\MonoGame.Content.Builder.targets" />
```

Thanks to @jamesford42 and @tomspilman for the help.