create Query class to allow a query to serve as a data source
refactor PG::Source to use HasDDL module
rename variable for clarity
change HasDDL module interface to use module method
formatting
remove unused Field class
DDL module raises an error if source result has duplicate fields
Copier uses variable named "source" instead of "source_table"
Copier uses variable "destination" instead of "destination_table"
Copier falls back to droppy is source is a query
Copier raises an error if using query as source for diffy/find_deletes
update readme for query as source
Merge pull request #6 from th7/feature/source_from_query

Feature/source from query
Revert "Merge pull request #6 from th7/feature/source_from_query"

This reverts commit 7967bb815caf87b2fe904ba6bab902a89ec13e23, reversing
changes made to 2bb0de8b9c182f605edc9d7ff9d6bca49c503256.
Copier uses variable named "source" instead of "source_table"
Copier uses variable "destination" instead of "destination_table"
rebuild ddl each time it is needed