Enable to not require mujoco at build time conditionally

[kanarus]

related to #23

[kanarus]

On the premise, current rusty_mujoco requires MUJOCO_DIR environment variable for:

    println!("cargo:rustc-link-search={mujoco_lib}");
    println!("cargo:rustc-link-lib=dylib=mujoco");

in the build.rs.

So, maybe we can change this to:

• make MUJOCO_DIR optional
• (only) when MUJOCO_DIR exists do println!s

[kanarus]

This may not be ideal: if rusty_mujoco's build script doesn't know the place of libmujoco, it can only ouput cargo:rustc-link-lib=dylib=mujoco with believing that libmujoco is in a common library path. This leads to runtime crash when libmujoco doesn't exist there actually (or its version is wrong), not a compile error.