Add more lua functions

[griffi-gh]

Added lua api functions (Mostly related to entities):

World:

• world:set_gravity(x: number, y: number)
• world:emit(g_id, [id]) --> entity (experimental)

Entity:

• entity:set_angle(angle: number)
• entity:set_angular_velocity(velocity: number)
• entity:is_hidden() --> boolean
• entity:set_fixed_rotation(enable: boolean) (experimental)
• entity:is_fixed_rotation() --> boolean (experimental)
• entity:set_gravity_scale(scale: number)
• entity:get_gravity_scale() --> number
• entity:apply_force(x, y, [point_x, point_y])

(for world:emit() - #109 should be merged first to prevent crashes)

I added checks for 1.5.1 because there's no level version code for 1.5.2 yet. fix this after 1.5.2 release.

[griffi-gh]

partially merged:

entity:set_angle https://github.com/Bithack/principia/commit/b030e27165592748f3eb337471886fd17149c865
entity:is_hidden https://github.com/Bithack/principia/commit/228841aaae7e66314e3bd8a1d1056561587b8af4
world:set_gravity https://github.com/Bithack/principia/commit/2c121ca597d97439a69e9b7874420f7b23236969