Added InvalidCommandException

Added InvalidCommandException:
- gets thrown when empty or invalid command is entered.

Added executeAdbCommand() method:
Executes an ADB command with specified command.
Can be used for shell/root command.
Example:
private void jButton1_Click(java.awt.event.MouseEvent) {
adb = new Adb();
adb.executeAdbCommand("install xyz.apk", false, false); /*Would execute
an ADB command without returning the output and not using shell/root
shell*/
}