Release v.2.1.0

• Window function support. Sample usage can be seen here.
• Database views support
  • Jet generator now generates view sqlbuilder and model types.
  • sqlBuilder types are generated in view folder and will behave the same as table sqlbuilder types.
  • model types are generated in model folder. To use view model type as grouping destination primary_key tag has to be specified manually. More information here.
• Jet internal types are now aliased. This allows passing of manually constructed expression slices as variadic argument of some operators and functions.
• [API break] ColumnList is aliased and reverted to be defined as slice of Columns. This will allow conditionally constructing the list of columns, by appending to a slice, before the statements.
  To fix build, just replace parentheses with braces. ColumnList(.....) -> ColumnList{......}