Filters improvement

Added range filter for grades
Removed most of magic strings
Modified information stored in DB
Improving code readability
Merge branch 'master' of https://github.com/barcicki/GorCalculator
Calculator fixes and Tests

Calculator fixes
Tests added thanks to wysek propsal
Switching from float to double (partial)
Added more tests and fixes

Tests for mid kyu and dan calculations
Fix the problem with restoring opponents