Add swapCase function

[Shekhar Gulati]

This function will have following signature

public static String swapCase(String input)

Test case should pass following tests

assetEquals(swapCase("AaBbCcDdEe"), "aAbBcCdDeE");
assetEquals(swapCase("Hello World"), "hELLO wORLD");
assetEquals(swapCase(""), "");
assetEquals(swapCase(null), "");