Added a test suite for the solow module.
Added a test suite for the solow module.

oyamad/QuantEcon.py,andybrnr/QuantEcon.py,andybrnr/QuantEcon.py,mgahsan/QuantEcon.py,QuantEcon/QuantEcon.py,chenxulong/quanteco,dingliumath/quant-econ,oyamad/QuantEcon.py,QuantEcon/QuantEcon.py,mgahsan/QuantEcon.py,gxxjjj/QuantEcon.py,dingliumath/quant-econ,agutieda/QuantEcon.py,jviada/QuantEcon.py,gxxjjj/QuantEcon.py,jviada/QuantEcon.py,chenxulong/quanteco,agutieda/QuantEcon.py