Fix library paths for BFGMiner

[gevans]

It looks like there are some issues finding libraries when starting BFGMiner:

$ pick use bfgminer
$ pick exec ...
bfgminer: error while loading shared libraries: libblkmaker_jansson-0.1.so.0: cannot open shared object file: No such file or directory

This can be fixed by prefixing exec commands with the library path:

$ LIBRARY_PATH=/path/to/pick/versions/current/lib pick exec ...

Pick should handle these paths and export the proper environment variables automatically.