Errors reported from Describe and Context blocks

Quick update to make terminating errors that occur in a Describe or Context block cause a failed "test" to appear in the output.

Because we're just piggybacking onto the existing PesterState.AddTestResult and Write-PesterResult code, the output looks as though it came from an It statement, though the name is always "Error occurred in Context block" or "Error occurred in Describe block".  If this style of output is undesirable, a more extensive code update would be required.
Test failure for script file errors (syntax, etc)

This is along the same lines as the original commit in this branch, but addresses issue #91.

Also moved the calls to Add-SetupAndTeardown into the Try blocks of Describe and Context.
Removed tab characters from previous commit

Whitespace-only change
Merge pull request #168 from dlwyatt/DescribeAndCheckoutFailureReport

Errors reported from Describe and Context blocks
Updated changelog for PR168.