Unity Package Manager Support

This release contains only one change: UPM support.

There is one breaking change: Unity versions below 2017.1 aren't supported from now on.

I won't build unitypackage files anymore. Please, install with UPM:

{
  "dependencies": {
    "st.rect-ex": "https://github.com/slavniyteo/rect-ex.git#v0.2.0"
  }
}