228 fix bug

### Fixed next issues:
1. `index_name_exists?` method to take into account table schema
2. duplication of tables in list for dumping schema file
3. table list for non-postgres users

### RSpec
```
155 examples, 0 failures, 2 pending
```
Coverage: 97.21%

### Docs
```
Files:          43
Modules:        40 (    0 undocumented)
Classes:         7 (    0 undocumented)
Constants:       8 (    0 undocumented)
Methods:       143 (    0 undocumented)
 100.00% documented
```