Datasiphon

- added helper method in `sql.SQL` backend - `is_query_paginable` - to check if query is paginable (for pointer based pagination) - based on selected columns

- Added support for parsing `datetime` and `date` objects in `sql.SQL` backend from `str` format (assuming iso-format of string)
- New class `FilterTypeParser` added to `sql.SQL` backend to return constructor for given type (e.g. `datetime`, `date`, `int`, `float`, `str`, `bool`) - now able to parse `datetime` and `date` objects from `str` format

- Validation changes: now instead of checking whether value in filter is of instance `column.type.python_type`, it uses the given `python_type` constructor to check if the value is processable by the given type
- Operations now apply the `python_type` constructor to the value before applying the operation

- Added advanced tests and logging

- Removed support for `ignore_extra_fields` and `strict` options in `sql.SQL.build` method
- added feature for infinite nesting - using `junctions` - AND/OR operators for sql queries - allowing advanced filtering and grouped clauses
- refactored validation of filter structure and columns, and restriction models
- added base class which has to be used when creating custom restriction models

- added option `strict` kwarg (defaults to `True`) to `sql.SQL.build` method to ignore fields in filter that don't correspond to expected `FORMAT` - does not apply to `ignore_extra_fields` option
- method `sql.SQL.validate_filter_structure` now instead of 'only validating' returns `dict` of processed filter items (because of `strict` option)