Implement versioning feature.

It is now possible to switch the version
and including 3.2.0

using(for 3.2.0):

```
$ mkdir lib/sinatra/assets/3.2.0/
deploy to the lib/sinatra/assets/3.2.0/*.*
```

```
class WebApp < Sinatra::Base
  set :bootstrap_version, "3.2.0"
  register Sinatra::Twitter::Bootstrap::Assets
  ...
end
```
Wipe out SHA1 values

Generated using the `Digest::SHA1`