Fixing broken imports after reorganizing them

See #6: http://github.com/meisam/YSmart/issues/issue/6
Moving sqltokens to the 'src' folder
renaming ysmart.py to sql2mapred.py

ysmart.py was causing name conflicts with the ysmart package and its
subpackages (ysmart.frontend, ysmart.backend)

See #6: http://github.com/meisam/YSmart/issues/issue/6
Updating the quick_start.txt file

See #6: http://github.com/meisam/YSmart/issues/issue/6