Support query params in Base#init, Base#fetch and Project#issues.

This patch adds the following functions:

- The extend and fields parameters can be specified
  for Base#initialize and Base#find.
- The extend, fields, startAt, maxResults can be specified
  for Project#issues.

It allows you to fetch the changelog of an issue as
follows:

issue = client.Issue.find('MYPROJ-10', {expand:'changelog'})
issue.changelog['histories'].each do |c|
 ...
end

It also allows you to specify the number of issues to be fetched
as follows:

issues = project.issues({startAt:1, maxResults:100)
Merge pull request #65 from buri17/feature-query_params_for_fetch

Support query params in Base#init, Base#fetch and Project#issues.