CLN: write the attributes in a HDFStore as strings

[jreback]

These strings are converted to bytes, previously these were written as unicode

[jreback]

no....that has to do with the format of the attributes and such (will bump if we ever need to actually change it); I think I did once between 0.10 and 0.10.1

This pr just fixed a display issue I think. was actually writing the unicode (rather than bytes) for pandas_type,pandas_version (because I decode them and the variable was being passed around). It actually doesn't make any difference and you would not even be able to know except if you do ptdump -av the (and you are not using PyTable 3.0.0), then these attribues look like string (e.g. 'appendable_frame'), while in PyTables 3.0.0 (even if you are not using it, but have the ptdump installed), they are bytes (b'appendable_frame')

somehow they were getting pickled (rather than written as strings), but this was all transparent (just a bit inconcistent that's all). You should be able to read/write something from an older version with a newer version (of PyTables) and vice-versa

if you try out 0.12rc1 you can see these (as compared to master)...install PyTables 3.0.0, write a store and do ptdump on it (then install master) and do the same thing