get_issues() to only return issues, not PRs

[pllim]

Fix #34 by limiting get_issues() to really only get issues, not PRs. Since PR handler has its own methods to get open PRs, this should not break anything?

>>> changebot.github.github_api import RepoHandler
>>> repo = RepoHandler('astropy/astropy')
>>> repo.get_issues('open', 'Close?')  # No 3845
[6025, 5193, 4842, 4549, 3951, 2603, 2232, 435, 383, 282]
>>> repo.get_issues('open', 'Close?', exclude_pr=False)  # Has 3845
[6025, 5193, 4842, 4549, 3951, 3845, 2603, 2232, 1920, 435, 383, 282]

Also added a test for this as per #33.

p.s. I briefly thought about having get_issues() to return a list of handlers instead of just numbers, but that has larger overhead, so will need a good reason to do so.

[coveralls]

Coverage decreased (-0.3%) to 67.172% when pulling 5e57d89 on pllim:get_issue_no_pr into 2e32dcc on astropy:master.

[coveralls]

Coverage increased (+1.3%) to 68.75% when pulling 2674b46 on pllim:get_issue_no_pr into 2e32dcc on astropy:master.

[coveralls]

Coverage increased (+2.6%) to 70.053% when pulling ce38ee5 on pllim:get_issue_no_pr into 2e32dcc on astropy:master.

[astrofrog]

This is great, thanks!