Pyswrve

* Added Items API support, see `SwrveItemsApi` class for details
* `get_item_lst` and `get_item_attrs` methods allow to request information about exist items
* `create_item` and `create_items` methods allow to create new items and update exist ones

This is minor release.

* `**kwargs` added in `SwrveExportApi` methods `get_kpi`, `get_evt` and others that can use additional parameters in requests (see swrve api docs). It allows to use parameters like `history` and `granularity`, setup `start`and `stop` manually for each request.
* New argument `as_datetime` in `SwrveExportApi` methods `get_kpi`, `get_kpi_dau`, `get_evt`, `get_evt_dau`, `get_payload`, `get_user_cohorts`, `get_item_sales`, `get_item_revenue`. When `as_datetime=True` dates in requests results are converted from string like 'D-2017-01-01' to datetime objects.
* Minor fixes.

* Big refactoring
* Added detailed docstrings
* Added tests
* Class `SwrveSession` replaced with new classes, base class `SwrveApi` and specific api-s classes `SwrveExportApi`, `SwrveUserdbApi` and `SwrveItemsApi`
* Added exception `SwrveApiException`
* Dropped garbaged functions and methods, dropped utils.py
* [`SwrveApi`] Added region (location) select: `us` or `eu`
* [`SwrveApi`] Renamed `save_defaults` => `save_config`
* [`SwrveExportApi`] Dropped `get_evt_stat` and added new methods `get_evt`, `get_evt_dau`, `get_payload`
* [`SwrveExportApi`] Dropped old `get_items_sales`and added new methods `get_item_sales`, `get_item_revenue`
* [`SwrveExportApi`] Renamed `ename` => `evt_name` in methods with events and payloads
* [`SwrveExportApi`] Removed `tax` arg and added `multiplier` arg in `get_kpi`, `multiplier` applies to revenue, arpu and arppu, like the revenue multiplier in Swrve Dashboard - Setup - Report Settings - Reporting Revenue
* [`SwrveExportApi`] Dropped `params` arg from methods
* [`SwrveExportApi`] Added `get_user_cohorts` method for [User Cohorts Export](https://docs.swrve.com/swrves-apis/non-client-apis/swrve-export-api-guide/User_Cohorts_Export)
* [`SwrveExportApi`] `set_dates` accepts `datetime` or `str` in `%Y-%m-%d` format
* [`SwrveExportApi`] Added `segment` arg to `get_kpi`, `get_evt`, `get_user_cohorts`, `get_item_sales` and `get_item_revenue` methods
* [`SwrveExportApi`] Added `currency` arg to `get_kpi`, `get_item_sales` and `get_item_revenue` methods
* etc