Attempted fix for CLang warnings in Vector3.h.

C++11 seems to have deprecated classes that have custom copy-constructor but not a custom assignment operator.
Implemented a true assignment operator for Vector3.
Fixed float comparison warnings in Vector3.

There's a bitwise comparison (Equals), and there's Eps-based comparison (EqualsEps).
Removed the controversial Vector3::operator <.

It hasn't been used in any C++ code and Lua doesn't need it.
Removed convert-assign operator.

We want all conversions to be explicit, not hidden.
Merge pull request #978 from mc-server/VectorAssignmentOperator

Vector3 warning fixes