Issue struct stores string key instead of int number

[jtuttle]

If we ever integrate with different issue tracking systems (like Jira), the key for an issue won't necessarily be a number like it is in GitHub. Jira issues, for example, are of the form ABC-123, so it will have to be a string. We should probably store the GitHub issue number as a string and just convert it to an int when needed to interface w/ the GitHub API.