Added indexesOf method

[iashok22]

indexesOf () - Finds the indexes of all the occurrences of given search key found in the given string.

[aaabramov]

Could you fix the formatting, please?

[vsTianhao]

Ha-ha，They refused to merge only two lines of code modification

[garydgregory]

The question here is whether to use the more grammatically correct "indices" or the more casual "indexes".

[kedar-joshi]

I think the method should return an empty collection instead of null

if(isEmpty(source) || searchKey == null ) {  
    return Collections.emptyList();
} 

[kedar-joshi]

Also, length of the source can be cached for better performance e.g.

for (int i = 0, length = source.length(); i < length; i++)
{
    if (searchKey.equals(source.charAt(i)))
    {
	indexList.add(i);
    }
}