Added generation mode for random input strings

MODE_ALPHABETS_ALL_UPPER_CASE
MODE_ALPHABETS_ALL_LOWER_CASE
MODE_ALPHABETS_MIXED_CASE
MODE_NUMERICS_WITH_ALPHABETS_ALL_UPPER_CASE
MODE_NUMERICS_WITH_ALPHABETS_ALL_LOWER_CASE
MODE_NUMERICS_WITH_ALPHABETS_MIXED_CASE

Default: MODE_ALPHABETS_ALL_UPPER_CASE