Evidence collection for each testcase

This patch adds feature to collect evidences for each testcase depending on users choice

What is Evidence? 
Command that is executed at target machine and result of same.

Changes:
To keep the changes minimal, I have overridden two methods of Class BaseTextFormatter 
1. dump_failures : It prints Evidence: instead of Failure:  and it will invoke base class methods irrespective of success, failure or pending testcases
2. dump_failure_info(example): Now this method will print command executed at backend, its result and backtracing information for each testcase instead of just failure.
- changes are made only in serverspec.rb file 

Note:
These functions will get overridden only if environment variable is set evidence=true

Execution:
After applying this patch 
Execute by following command
    
    $ rake spec evidence=true

After applying this patch serverspec execution will print the the command executed at backend and its output for each testcase(failed, success)

Please suggest any comments/improvements for this patch.