Corrections and documentation on String

Corrections and documentation on String

* Removed "made-by" comment that shouldn't be there according to the coding
  style guidelines
* Removed unused includes
* toUpper() and toLower() now uses contextual case changing instead of
  changing each character individually
* Floating point conversions now check for a lot more things (from and to)
* String::String(const char) replaced by String::String(const uchar)
* String::utf16() now correctly returns a copy of the string data
* String::merge(String) now correctly merges all instances of a String