changes to 'Generator utils: added tests, broke up rand_in'

rand_in() now only returns a random element in an array.
For a random integer between two values, use rand_between().

--HG--
rename : Generator.rb => lib/Generator.rb
Added tests for StringGenerator
Refactored @null and @unique as decorator classes

Created UniqueGenerator and NullGenerator classes. Created
tests for UniqueGenerator.
Moved Column.rb and Table.rb into lib directory

--HG--
rename : Column.rb => lib/Column.rb
rename : Table.rb => lib/Table.rb
Fixed unique and null in EnumGenerator and Table
A little clean up in Generator.rb