Added DataFrame>> #toString. Fixed #126.

[Joshua-Dias-Barreto]

I've implemented a method toString so that DataFrames can be converted to a string table.

	df := DataFrame withRows:
		      #( #( 'Barcelona' 1.609 true ) 
                         #( 'Dubai' 2.789 true )
		         #( 'London' 8.788 false ) ).

	df rowNames: #( 'A' 'B' 'C' ).
	df columnNames: #( 'City' 'Population' 'BeenThere' ).

df toString.

This should return a string which looks like a DataFrame

#    City         Population  BeenThere  
'A'  'Barcelona'  1.609       true       
'B'  'Dubai'      2.789       true       
'C'  'London'     8.788       false