PostgreSQL - Later insertions fail after sample data was installed

#### Steps to reproduce the issue

Install sample data.

After this inserting a new article will fail. because it will try to insert it with id=1 (sequence was not incremented). PostgreSQL works a bit different from MySQL.

#### Expected result

No issues inserting new data after sample was installed.

#### Actual result

Fatal SQL errors because of duplicate keys.

#### System information (as much as possible)

PostgreSQL 9.3

#### Additional comments
PK fix for sample_data.sql
Fixed sample_learn.sql
Fixed errors while inserting data.

Some queries fail in PG 9.3 because they are not properly escaped.
Merge pull request #3294 from vdespa/patch-7

PostgreSQL - Later insertions fail after sample data was installed