add simplecov coverate reports
add feature for single currency to SchemaStatements and refactored specs

add specs for single_currency to table statements

renamed the test names to hopefully make it more readable

also renamed the migrations used in tests for readability.
their class names are the same if they are functionally
equivalent, but are organized in different modules depending
on whether it is implemented using
schema_statements, create_table or change_table
update README with migration helpers usage