NEW: lang/gravity

Hi ports --

Attached is a new port, lang/gravity. Gravity is a small, high
performance language with a modern syntax.

---
pkg/DESCR:
Gravity is a powerful, dynamically typed, lightweight, embeddable
programming language written in C without any external dependencies
(except for stdlib). It is a class-based concurrent scripting language
with modern Swift-like syntax.

Gravity supports procedural programming, object-oriented programming,
functional programming and data-driven programming. Thanks to special
built-in methods, it can also be used as a prototype-based programming
language.
---

Tested on amd64 and octeon. All build warnings are upstream's use of
some C11 constructs (compiling with clang -std=gnu11 eliminates all
warnings) but upstream is aware and plans to make the codebase C99.
There are a few failed tests, but upstream says those are expected.

~Brian