Fix RUST_SRC_PATH on Windows

Windows uses a semi-colon (`;`) rather than a colon (`:`) for separating paths due to drive letters. Currently, racer changes a path like `'C:\rust\src'` into `['C', '\rust\src']` (see #47). This may work or it may not (I don't know the rules for paths without drive letters, but it does work for some situations). 

I added a check for `#[cfg(windows)]` and `#[cfg(unix)]` to switch between using `;` and `:`.