irrString.h improvements and new features/functions.

This patch adds two new functions:
append(const T* const other, u32 length), for specifying the desired amount of characters to append.
replace(const string<T,TAlloc>& toReplace, const string<T,TAlloc>& replaceWith), for string replacement.

It changes remove() to pass by reference instead of by value.
It adds some size() == 0 checks to abort functions early.
It changes most of the functions to return *this to allow function chaining.