Dbml

- Fix: files in **UTF-8 with BOM** encoding couldn't be parsed.

- More tests and more bug fixes.
- Added index columns validation.
- Added table group items validation.
- References now contain link to Table and Column objects instead of just names.
- Indexes now contain link to Column objects instead of just names.

- Better syntax errors.
- sql for each object now contains in `sql` property instead of string rerpresentation. Added proper string representations.
- Added syntax tests.
- Million bugs fixed after testing.

- Comments are now parsed too if they are before [b] or on the same line [l] as the entity. Works for: tables[b], columns[lb], references [lb], indexes[lb], enum items [lb], enums [b], project [b] and table group [b]
- All class instances will now have an empty Note in `note` attribute instead of None.
- Add string representation for Note and EnumItem.
- Enum instance now acts like list of EnumItems.
- Add EnumType to use in column.type attribute.
- Column type is now replaced by EnumType instance if enum with such name is defined.
- Remove unnecessary ColumnType class.
- Fix: note definition, project definition, some other definitions

- Initial release