more code optimization

more code optimization and refactoring
job_create method optimization

job_create method optimization and code refactoring
added checks to Job model methods

Now methods of Job model checks if job is exists
added checks for Balance model methods

added checks if object exists (after getting object by
select_for_update) for Balance model methods
code refactoring for show_job method
code refactoring
code refactoring
added title methods for Settings

added get_site_title method for Settings;
added set_site_title method for Settings;
tax value is set as int by set_tax_value method (was Decimal before,
little bug)
fixed bug
fixed bug
added logo image and changed link to settings
added link to settings
implemented Site Settings view and form
changed site_title to get_site_title method