Add a IssueKey class to support extracting projectKey and issueId values from an issue key.

This is a necessary first step to supporting validation of the issue's
project key seperately from the full issue key.

Issue: #7
Extract the static inner InvalidIssueKeyException class from IssueKey.
Add support for skipping validation of issues that do not match a known JIRA project key.
Add missing log statement.

Without this change, any emitted response exception was not logged.
Add a configuration UI checkbox for the new ignoreUnknownIssueProjectKeys setting.
Handle the 404 responses emitted for missing project keys.
Exclude issues with invalid project keys from all validation.

This fixes a bug in the previous implementation, whereby a commit would
be treated as valid due to validation not being performed on issues with
an invalid project key, but the list of matched issues being non-empty.

Since the list was non-empty, the check for *any* matched issue
succeeded, and the array of errors was empty: the commit was considered
valid.
Merge pull request #8 from landonf/issue-7-project-key-validation

Issue #7: Project Key Validation