ENH: allow axis argument to append / move append code to generic.py

http://stackoverflow.com/questions/25896241/creating-columns-dynamically-assigning-them-a-constant-row-vector/25896504#25896504

e.g. 

Instead of 
`pd.concat([df,DataFrame([[1,2,3,4]],columns=list('ABCD'),index=df.index)],axis=1)`

allow as a convience feature
`df.append(DataFrame([[1,2,3,4]],columns=list('ABCD'),index=df.index),axis=1)`
