Introduce postInstallMirrorTwitterBootstrap scripts

The "mopa:bootstrap:symlink:less" console command has a "--no-symlink" option, which can be used to make mirrors instead of symlinks.
But nothing has been made for the composer postInstall scripts. With this PR, you can now use 
"Mopa\\Bundle\\BootstrapBundle\\Composer\\ScriptHandler::postInstallMirrorTwitterBootstrap"
instead of 
"Mopa\\Bundle\\BootstrapBundle\\Composer\\ScriptHandler::postInstallSyminkTwitterBootstrap"
in "post-install-cmd" && "post-update-cmd"

I also think all this part should be refactored to avoidcode duplication between "Command" and "Composer" classes.
Why not a "ScriptHandler::InstallBootstrap" && "console mopa:install:bootstrap" with sass/less related options and default to mirror instead of symlink (like symfony does)