Integrate significance of motif differences

[Treutler]

• implement function for the calculation of a p-value for one motif position of one motif pair
• calculate p-values for each motif position for each motif-pair
• display significances indicators in difference logo / difference logo table (suggestion: * for p < 0.05, ** for p < 0.01, *** for p < 0.001)
• display legend for significances indicators in difference logo / difference logo table