Fixed #272 - added picker for specific month and year

...locker8/budgetmaster/controller/DatePickerController.java

.../de/deadlocker8/budgetmaster/services/HelpersService.java

src/main/resources/static/css/style.css

src/main/resources/static/js/categories.js

src/main/resources/static/js/datePicker.js

src/main/resources/static/js/payments.js

src/main/resources/templates/datePicker.ftl

src/main/resources/templates/index.ftl