Support extra params for "Walker" class and "Api::getIssue", "Api::search" methods

[aik099]

Problem

1. the Api::getIssue method supports expand parameter, but doesn't allow specifying other parameters of the underlying Jira REST API call (fields, fieldsByKeys, properties, updateHistory, failFast)
2. the Api::search method supports jql, maxResults, fields and nextPageToken parameters, but doesn't allow specifying other parameters of the underlying Jira REST API call (expand, properties, fieldsByKeys, failFast, reconcileIssues)
3. the Walker class suffers from the same customization problems as the Api::search, that is used internally

Solution

1. add the array $params = array() parameter to the Api::getIssue, Api::search and Api::search methods and combine $params array built in the method itself with a provided one
2. for the Walker class:
3. add the protected $params property of the array type
4. add the array $params = array() parameter to the Walker::push method and store it in the $params class property
5. specify $this->params to the executed Api::search method in the Walker::getIterator method
6. adjust tests to confirm that extra parameters (just test couple of the parameters) are forwarded into the underlying Jira API calls

Closes #191