warnings when compiling in windows #75

I am compiling in Windows (VS2017) and I am getting warnings like

tmxpropertyset.h(66): warning C4267: 'return': conversion from 'size_t' to 'int', possible loss of data

Basically it shows a warning for all the functions that return size of a stl container, since the return value is cast to an int. Technically, size() returns a size_type.

I was going to create a PR changing all the interfaces that get the size of a container to return a size_type, but I wanted to ask for your opinion first, @sainteos