Add Travis CI
Use `lua_pushnumber` to push int64 on Lua < 5.3

The problem occurs when lua_Number is double and lua_Integer is int32
In this case `mp.unpack(mp.pack(0xFFFFFFFF)) == -1`

(Originally each sizeof() check was for sizeof < 64, which didn't make sense.
 Committer changed it to check for sizeof == 4 instead.)
Allow LUACMSGPACK_MAX_NESTING to be set by CPP
Add tests for more edge cases

The 0xFFFFFFFF test can potentially fail on 32 bit systems if
our internal conversions are casting to wrong Lua types.
lua_Integer != lua_Number