URL building and filters handling improvements

Changelog:

- rename `processManagementData` to `getMGMTData` like as others rga functions;
- optimize `getMGMTData` function for less `paste` and `.self$getToken` calls;
- change build URLs logic according `httr` package: use `modify_url` function with `query` arg;
- change default `start.date` and `end.date` args in `.self$getData` and `.self$getMCFData` function: from "today" to "last week";
- improve filters handling: now you can use whitespaces and `OR`, `AND`, `||`, `&&` operators:
  - `ga$getData(83639328, filters = "ga:users < 3000 && ga:pageviews < 10000")`;
- apply `RCurlOptions` options only for Windows.

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wbr,
Artem