Migrate all issues and ignore pull requests

[t1bb4r]

Very useful library!

I wanted to use this library to 'properly' clone another repository. I encountered the following challenges:

1. Open pull requests were being created as issues. Since pull requests are branch specific issues and some branches might be deleted, I feel it's better in general to just ignore all pull request issues.
2. Closed issues weren't being merged. Closed issues is an very important part of the repository for me, so I decided to add these.

If you would like to keep the library as is, I could add an options parameter to the client:

#@params options [Object] set of options
#@option options [String] :state (open) State: <tt>open</tt>, <tt>closed</tt>, or <tt>all</tt> 
#@option options [Boolean] :pull_requests (true)
def initialize(username, password, source_repo, target_repo, options)