Suggestion/Discussion: read_hdf
At the moment `read_hdf` is implemented such way that it opens HDFStore connection immediately when it is called. If the file does not exist, then it creates an HDF file and then throws `KeyError`. There are two side effects:
- You can not distinguish situation when the file does not exist from situation when the key does not exist in the file
- If you have a typo in the filename, empty file is created -> polluting directory 

I suggest to check if the file exist before and throw `IOError` if it does not.
