Implements IDice interface with two 6-sided dice

The TwoD6 class introduced in this commit simulates two 6-sided dice,
using random numbers between 1 and 6. Unit tests are provided in the
corresponding JUnit test class.
Improves TwoD6 unit tests

Adds a test for isDoubles(). Additionally, the tests for roll() now
check if the result is >= 2 rather than >= 1.
Adds javadoc for TwoD6Test.testIsDoubles()
Merge pull request #1 from JLimperg/master

Implementation of IDice interface