13274 rails4 

Task 13274 - move pg_power to Rails4

## Changes
* Update `Gemfile` to depend on Rails4
* Update `add_index` method according to rails4 changes(no need support for `:where`).
* Rename `extensions` method to `pg_extensions` (since rails4 introduced also `extensions` method)
* Enhance `spec` task to build test database
* Address deprecation warnings
* Add note about Rails3 to README

Branch `rails4` is moot and should be removed.

## Rspec

```
Finished in 0.98795 seconds
90 examples, 0 failures, 3 pending
```

## Documentation

```
Files:          42
Modules:        39 (    0 undocumented)
Classes:         7 (    0 undocumented)
Constants:       8 (    0 undocumented)
Methods:       133 (    0 undocumented)
 100.00% documented
```