Add logging

[mgsloan]

It'd be nice to be able to add warning / info messages to ide-backend. Two recent cases where this would be helpful:

1. Warning the user when GHC_PACKAGE_PATH is being unset (see GHC_PACKAGE_PATH is problematic for usage with stack #295 / GHC_PACKAGE_PATH not visible from running code #294).

2. Warning when there might be an ide-backend / ide-backend-server mismatch Better error messages for ide-backend / ide-backend-server mismatches #292

One thing to note is that some ide-backend users assume it's silent. In particular, stack-ide uses stdout, with a particular protocol, for communication.

[mgsloan]

Done!