Added omit tag for column names.

When parsing the sql_column name tag, pan will now ignore any field
tagged with "-". This is to allow structs to have properties that aren't
stored in the database.
gofmt and added omit test.

Restore 100% test coverage by testing for omission of columns.

Run gofmt.
Merge pull request #7 from DramaFever/paddy_omit

Added omit tag for column names.