Make connection.tables method return tables from all schemas. Issue #18

Task 13289 - address pg_power open issues

This PR addresses https://github.com/TMXCredit/pg_power/issues/18

## Changes

* Redefine adapter's `#tables` method to return tables in all schemas
* Fix broken specs (now instead of `tablename` there are `public.tablename`).
* Write spec to test `#tables` method.
* Add missing docs

## RSpec

```
Finished in 0.77275 seconds
90 examples, 0 failures, 3 pending
```

## Doc
```
Files:          42
Modules:        39 (    0 undocumented)
Classes:         7 (    0 undocumented)
Constants:       8 (    0 undocumented)
Methods:       133 (    0 undocumented)
 100.00% documented
```