Fixed issue of incorrect severity mapping with Resharper Issue has its own severity different from the IssueType severity

[BrentRodhouse]

Addresses #14

Resharper reports can have Severity defined on both the IssueType and the Issue. In cases where the Issue's Severity differed from the IssueType's severity, resharper-to-codeclimate incorrectly mapped the IssueType's Resharper Severity to the CodeClimate issue severity. This PR addresses this discrepancy.

Changes:

1. CodeClimate severity is now populated from the Resharper Issue Severity if it exists and falls back to the previous behavior of obtaining the Severity from the IssueType if it does not.
2. I added a new test case to test scenarios where Issue Severity and IssueType Severity are different.
3. I tweaked RunToolTest.cs so that it can accept multiple test cases. Expected output was moved from being hardcoded to a JSON file. Comparisons between expected and actual output are no longer susceptible to formatting discrepancies. Further test cases can now be added by adding a line to the .cs file and adding an input file and an expected output file.