pandas unpickling string nan values
Hi.  I'm using Pandas 0.11.0.  I have a table with some string None values which I both pickled and appended to an HDFStore (compressed with blosc), and then retrieved with pandas.load and HDFStore.get.  In the unpickled table, string values are converted to the string "nan", which does not get detected by isnull.  On the other hand, the HDFStore correctly retrieves NaN values.  

I'm using natural language data and there is a slim but non-zero chance that "nan" (as a string) is a value representing actual data, and even worse, I didn't know that the unpickled table even _had_ None values until I tested out the HDFStore.  

This is my first github bug report, and I don't know if this is a bug or intended behaviour, but it seems bug-like to me.  
