Refactored and enhanced installation process.

## Ready State: Yes

* Added ImageX InstallKit specific Exception class: `ImagexInstallKitException`
* Added ImageX InstallKit install task interface: `ImagexInstallKitInstallTaskInterface`
* Refactored ImagexInstallKitInstallTask to implement interface.
* Refactored install task bootstrap `imagex_installkit_install_bootstrap` to re-define `$tasks` stack to separate modules and non-module (dependency) installation tasks.
* Refactored task execution handler and renamed.

## Deployment

* Merge code.