Describe / Context error output fix

Previous code was relying on the ScriptStackTrace property of ErrorRecord objects, which doesn't exist in PowerShell 2.0.  Code updated to use ErrorRecord.InvocationInfo.PositionMessage instead.
Merge pull request #186 from dlwyatt/DescribeAndContextErrorOutputFix

Describe / Context error output fix
Update CHANGELOG.md
Adding some examples (which are really tests)
Adding Gherkin Support to Pester
Add support for background and Scenario Outline
Create my own (mini fork) of pickle for simpler parsing
Refactor Features and Steps to their own functions for reuse
Refactor WHEN to just store steps in a hashtable
Fix tag filtering (it's still not feature-compatible with Cucumber's)
Allow multiple matches and pick the best match
Fix Background (should (re)run before every test)
Add the keyword to the step printed output
Blindly updating wrapper code from changes to Pester Beta ;-)
Some more output tweaks for Gherkin results
Add support for tables in steps
Fix code coverage for Gherkin
Add binaries (for now). Really we should just generate these as part of a build (pull from NuGet).
Fix bug in Path parameter (we still need to load ALL steps files)
Fix bug in step matching: the pattern should match the WHOLE name from ^ to $
Fix DocString parsing/matching so docstrings are just the last parameter
This allows steps with (.*)? regex to match a string or receive a docstring at the end.
Make scenario-name matching work