`load_hdf5()` filename issues

If `Context.load_hdf5()` is called with a relative path, it may fail with an:

``` python
IPython.parallel.error.CompositeError: one or more exceptions from call to method: func_wrapper
[0:apply]: IOError: Unable to open file (Mpi_err_no_such_file: no such file or directory)
[1:apply]: IOError: Unable to open file (Mpi_err_no_such_file: no such file or directory)
[2:apply]: IOError: Unable to open file (Mpi_err_no_such_file: no such file or directory)
[3:apply]: IOError: Unable to open file (Mpi_err_no_such_file: no such file or directory)
```

No error results if an absolute path is passed in instead.

I'd like to see `Context.load_hdf5` detect relative paths client-side and resolve into absolute paths before passing to engines, which should resolve this issue.