Fixes a bug pointed out by @weaversa:
https://github.com/GaloisInc/cryptol/issues/127#issuecomment-64464455
In addition to the other search path changes in #127, we now will add
the directory containing files to be loaded to the search path. This
applies to:
- files loaded with a command line argument, like in the original
comment
- arguments to `:l`, so for example `:l examples/DES.cry` would work
- batch file arguments, so for example running `cryptol -b
/some/path/bar.cry` adds `/some/path` to the search path.
