Automatically pull rust project location from RUST_SRC_PATH

[Susurrus]

It's part of the instructions for installing racer, so people should have it set automatically. Maybe pull in this environment variable if it exists and auto-populate the rust source textfield? It'd be nice if I could just have one setting for this repo and it'd propagate to racer and gracer.

[isamert]

It checks for RUST_SRC_PATH variable. If you run gedit from terminal you can see that it will automatically find rust source path. But I dont know why, if you run gedit from its shortcut, python cant find the RUST_SRC_PATH variable, if you specified this variable in your ~/.bashrc file. If I can find the reason, I'll fix this.