v0.3 fleur de douche

v0.3 brings several improvements:

• join operation is now available on filters sharing the same parameters #12,
• a function for freeing memory has been added: void fleur_destroy_filter(BloomFilter * bf)
• fleur was closing file descriptorss on errors in several functions. This behavior has been changed: it's now up to the caller to define what to do with the fd on errors.
• error messages have been harmonized.