Added the option that the user determines which SecureRandom object to
use. 
This is done by adding constructors that accept a SecureRandom object
along with the other parameters.
The random object is kept by the dlog group and used when needed.
Deleted two unused native functions.
Fixed bug in the encode byte array to GroupElement function. 
The given byte array is used to generate an element. The generated
element is always a valid element and thus, there is no reason to check
it again. 
Changed the boolean indicates whether check the element to false.