13323 rails4

Task 13323 - make it work with rails 4

## Changes
* Get rid of `protected_attributes` test dependency for rails 4. Don't use mass assignment. In rails 3 we could use `attr_accessible` method to do mass assignment, in rails 4 we don't have it, it was extracted to `protected_attributes`. But `protected_attributes` doesn't work with rails 3 because of its rails4 dependency. To avoid this hell I've decided not to use mass assignments in few places.
* Update rails dependecy in Gemfile from `~> 3.1` to `>= 3.1`. That allows people to use Themis with rails 4
* Regenerate gemspec, bump to version 0.2.0.

## RSpec

Pass for all Rails3.1, Rails3.2, Rails4.0: https://travis-ci.org/TMXCredit/themis

## Doc

```
Files:          14
Modules:         9 (    0 undocumented)
Classes:         7 (    0 undocumented)
Constants:       1 (    0 undocumented)
Methods:        36 (    0 undocumented)
 100.00% documented
```