Configurable play build executable

 * Made play build executable configurable to facilitate Play 2.3 activator/sbt changes

Added configuration option for the name of the executable for building the play project (which ultimately invokes SBT).  Prior to Play 2.3 the executable name was simply `play`.  From Play 2.3 onwards, the executable name can be either `activator` or simply `sbt` directly. This value is defaulted to `play` for backwards compatibility.  The configuration option is simply set like so:

```xml
   <executableName>sbt</executableName>
```