Feature: evergreen runner is able to run separate files or directories of files

That's said:
`evergreen run spec/javascripts/foo_spec.js`  will execute only one foo_spec file
`evergreen run spec/javascripts/models/`  will execute all specs in models directory

All this in the spirit of this [feature proposal](https://github.com/jnicklas/evergreen/issues/42)

Let me know what else should be done/changed? Thanks in advance.

#### Changelog:
- cwd is assumed as an Evergreen root
- Evergreen::Runner now receive first obligatory argument - list of files/dirs
- Evergreen::Suite.new now receive files (default is [])
- Evergreen::FilesHelper module added with methods to find specs/templates
- Specs added to suite_spec and files_helper_spec created