Add task for all files.

Hi.
I add rule into `Rakefile` now you can tests one files or one dirs.
For example

```
rake spec:core #run all specs into spec/core directory
rake spec:core:array #run all specs into spec/core/array/ directory
rake spec:core:array:allocate #run only allocate_spec.rb intro spec/core/array
```
And it for all dirs into `spec` dir.

When file or dir not found, then raised error.