Add. `safe` and `unsafe` module.

`safe` means that function returns nil and error object in case of libuv error
`unsafe` raise Lua error

```Lua
-- safe by default
-- could be changed at compile time
uv = require "lluv"

-- unsafe module
uv_u = require "lluv.unsafe"

-- safe module
uv_s = require "lluv.safe"

uv:tcp():bind("127.0.0.1",  5555);

-- return nil and error
uv_s:tcp():bind("127.0.0.1",  5555);

-- rise Lua error
uv_u:tcp():bind("127.0.0.1",  5555);
```