fixed #columns sql statement in FbAdapter to handle negative scale in decimal columns
columns parameters only set when needed in FbColumn#initialize
decimal scale gets converted to positive in Fbcolumn#initialize
decimal scale gets converted to positive in Fbcolumn#initialize
added position option to add_column
Merge pull request #14 from vizcay/master

improve FbColumn#initialize so that limit, precision & scale get set only for correct column types