Lua sector lines

Adds support for "sector.lines" in Lua, an array containing all the linedefs in a particular sector variable:

#sector.lines returns the number of lines in the sector.

sector.lines[ i ] (e.g. sector.lines[0], sector.lines[1], sector.lines[2], etc ....) gives you individual linedefs in the sector. If the number you supply is equal to or greater than #sector.lines, this returns nil.

Test script for your benefit: (see comments)

Type "luatest" in console in any level and you'll get a few print messages that tests these features for sectors[0]