Case sensitive keys and section names

[mikhail-sakhnov]

Now in the library keys and section titles processed with strings.ToLower()

It provides some strange behavior with keys like "testKey" - it would provide look up for key "testKey" in dict but it's been stored as 'testkey'.

It would be better if you either remove ToLower in func (dict Dict) add(section, key, value string) or add ToLower in Get* methods.