Case-insensitive keys and allowing a dot as section separator

[bleutzinn]

Keys are now case-insensitive making the matching of keys in the language files and in code more forgiving regarding differences in upper- and lowercase writing.

Also when using the helper function a dot . may be used as a section separator, for example echo L("category.somethingother");. As you are well aware this will not work when using the constant notation as in PHP the dot is not a valid character in constant names.

Updated the ReadMe file accordingly.