implement DataFrame methods

please fill everything thats needed @everyone, @luenensc, @schaefed

• implemented
• todo

methods (alphabitical):

• all()
• any()
• apply()
• astype()
• combine()
• combine_first() see #15
• copy()
• dropna()
• drop() see #26 (closed)
• x in dios
• is
• isin()
• isna()
• isnull() see 7ba46d49
• iteritems() see 2ce3840a
• iterrows() see 2ce3840a
• join()
• len(dios)
• mask() see 8264b254
• max() see 7ba46d49
• memory_usage()
• merge()
• min() see 7ba46d49
• notna()
• notnull() see 7ba46d49
• shape()
• squeeze()
• to_string()
• to_csv() see 7ba46d49
• where() see 8264b254

other:

• isempty() see 2ce3840a
• dropempty() see 2ce3840a
• hasnans() see 2ce3840a
• isdata() see 2ce3840a
• to_dios() see 75592cc0