Teach scc merge commands to work with repos without issues

Close #131.

This PR should minimally allow `scc merge` and `scc travis-merge` to stop erroring when executed in repositories where GitHub issues are disabled.

Can be tested with https://github.com/snoopycrimecop/snoopys-sandbox and https://github.com/snoopycrimecop/snoopys-sandbox/pull/1

```
sebastien@jrs-macbookpro-25399:snoopys-sandbox (dev_4_4) $ python ../snoopycrimecop/scc/main.py merge dev_4_4
2014-06-30 12:13:59,197 [   scc.merge] INFO  Merging Pull Request(s) based on dev_4_4
2014-06-30 12:13:59,197 [   scc.merge] INFO  Including Pull Request(s) opened by any public member of the organization
2014-06-30 12:13:59,198 [   scc.merge] INFO  Including Pull Request(s) labelled as include
2014-06-30 12:13:59,198 [   scc.merge] INFO  Excluding Pull Request(s) labelled as exclude or breaking
2014-06-30 12:13:59,927 [   scc.merge] INFO  Repository: snoopycrimecop/snoopys-sandbox
2014-06-30 12:13:59,927 [   scc.merge] INFO  Excluded PRs:
2014-06-30 12:13:59,928 [   scc.merge] INFO    # PR 1 sbesson 'Teach scc merge how to deal with repositories without issues' (user: sbesson)
2014-06-30 12:13:59,928 [   scc.merge] INFO  Already up-to-date.
2014-06-30 12:13:59,928 [   scc.merge] INFO  
2014-06-30 12:13:59,928 [   scc.merge] INFO  
sebastien@jrs-macbookpro-25399:snoopys-sandbox (dev_4_4) $ TRAVIS_PULL_REQUEST=1 python ../snoopycrimecop/scc/main.py travis-merge --info
2014-06-30 12:17:38,608 [scc.travis-m] INFO  Listing Pull Request(s) based on dev_4_4
2014-06-30 12:17:38,608 [scc.travis-m] INFO  Repository: snoopycrimecop/snoopys-sandbox
2014-06-30 12:17:38,608 [scc.travis-m] INFO  
sebastien@jrs-macbookpro-25399:snoopys-sandbox (dev_4_4) $
```

`scc rebase` is using the `is_merged()` method. So far it was using the undecorated `PullRequest.__getattr__` but I got at least one SSL error recently. So I added a decorated `is_method` to `PullRequest`. Slightly unrelated to this PR but should be safe.