implements and fixes #997

implements and fixes #997 
- ServerPaths object can be populated by passing base path to OSRM
- reduces code/functionality duplication in node-OSRM

cc: @jfirebaugh 
Thought it was initialized in

```C++
OSRM_impl::OSRM_impl(const ServerPaths &paths, const bool use_shared_memory)
    : server_paths(paths)
{
```

but it's probably not needed anywhere else besides the c'tor,