Added caching control for `i-api-request`

To disable caching of  'GET' request need to call method `get` with `cache: false` option:
```javascript
BEM.blocks['i-api'].get('path/to/resource', {cache: false});
```
Default caching of 'GET' requests on client side may be disabled, if custom realization of method `_getDefaultOptions` of `i-api-request` block returns `cache: false` option:
```javascript
// content of i-api-request.js:
BEM.decl('i-api-request', null, {
    _cache: false
});
```