Unified workflow with empty string and null in setAttributes and setAttribute
 - Added method convertBlankStringIntoNullInAttrValue into AttributesManagerBl
 - Modified methods setAttribute and setAttributes in AttributeManagerEntry
   - convertBlankStringIntoNullInAttrValue convert empty string into null for checkAttributeValue and checkAttributeExists() to unify work with empty string and null
 - Some database are not able to recognize empty string and null. This is the reason why we convert empty string into null in entry.
Description of tables and columns added

- use postgre comments in postgre.sql (-- instead of #)
- Locale columns
- add description for all tables in postgre.sql
- alter tables:
ALTER TABLE pn_receiver ADD locale varchar(512);
ALTER TABLE pn_template DROP COLUMN locale;
ALTER TABLE pn_pool_message DROP COLUMN locale;

-alter destination table: (add propagatio_type)
alter table destinations add propagation_type varchar(10) default 'PARALLEL';
Merge pull request #260 from zoraseb/table_documentation

Description of tables and columns added
Merge pull request #229 from papperwing/BlankString

Unified workflow with empty string and null in setAttributes and setAttribute