Add execute_{bash,csh,perl,python,ruby}_script matchers

Those matchers check whether a script was run via the
script resource or one of its shortcuts.

Example: chef_run.should execute_bash_script 'name-of-script'
Document script matchers in README.md
Test all script matchers
Merge pull request #97 from mlafeldt/script_matcher

Add execute_{bash,csh,perl,python,ruby}_script matchers