pandas.*.ix is depreciated

[khaeru]

/home/khaeru/vc/iiasa/ixmp/ixmp/core.py:730: DeprecationWarning:                                                        
.ix is deprecated. Please use                                                                                             
.loc for label based indexing or                                                                                         
.iloc for positional indexing                                                                                            
                                                                                                                      
See the documentation here:                                                                                       
http://pandas.pydata.org/pandas-docs/stable/indexing.html#ix-indexer-is-deprecated                                     
  jPar.addElement(to_jlist(key.ix[i], idx_names),

[khaeru]

Resolved by #105.