Allow to copy the profile

This is my attempt to fix #50. Currently it doesn't work yet:
```
ondrej@hawk:~/repos/python-hpcmp2(install)$ ./update -c some_dir
Up to date: launcher
Up to date: zlib
Up to date: python
Up to date: cython
Up to date: distribute
Up to date: docutils
Up to date: freetype
Up to date: szip
Up to date: hdf5
Up to date: pyzmq
Up to date: readline
Up to date: tornado
Up to date: ipython
Up to date: jinja2
Up to date: numpy
Up to date: png
Up to date: matplotlib
Up to date: mpich
Up to date: mpi4py
Up to date: nose
Up to date: numexpr
Up to date: petsc
Up to date: petsc4py
Up to date: pygments
Up to date: pytables
Up to date: scipy
Up to date: sphinx
Up to date: superlu
Up to date: sympy
Up to date: tetgen
Up to date: profile
Linking profile/jf5c into some_dir
[profile/jf5c] Nothing to do
[ERROR] Uncaught exception:
Traceback (most recent call last):
  File "/home/ondrej/repos/python-hpcmp2/_hashdist/hashdist/cli/main.py", line 136, in help_on_exceptions
    return func(*args, **kw)
  File "/home/ondrej/repos/python-hpcmp2/builder/builder.py", line 217, in main
    args.copy, virtuals, hdist_config)
  File "/home/ondrej/repos/python-hpcmp2/_hashdist/hashdist/core/profile.py", line 85, in make_profile
    with open(profile_json, 'w') as f:
IOError: [Errno 2] No such file or directory: 'some_dir/profile.json'

This exception has not been translated to a human-friendly error message,
please file an issue at https://github.com/hashdist/hashdist/issues pasting
this stack trace.
```

@dagss --- any ideas how to use the make_profile() properly? I think I might be constructing the artifacts argument incorrectly.