Add better `inspect` output for ExampleGroup

Changing from defining a singleton method to storing the inspect output on the ExampleGroup

Add documentation for instance_variables and inspect

Handling string element in instance_variables for ruby 1.8

Removing un-needed expct calls in spec

Handling ExampleGroup without inspect output, handling ExampleGroup without a description, refactoring logic to ignore `@__inspect_output` when determining before context ivars

Improve readability of no inspect output spec and does not copy inspect output from before context to examples spec
Merge pull request #1687 from kcdragon/feature-better-inspect-output-for-example-group

Add better `inspect` output for ExampleGroup