feat(changelog): Added feature to generate CHANGELOG.md

Could be done with tasks

`grunt changelog`

or

`grunt release:prepare`
Added forgotten call `grunt.loadNpmTasks('grunt-conventional-changelog');`
refactor(Gruntfile): to be able  handle 'grunt before-test' task
feat(Gruntfile): Added added task ,checks if branch is clean
feat(Gruntfile): Added task 'grunt release:push'
feat(Gruntfile): Added empty CHANGELOG.md
feat(Gruntfile): Added helper task isGhPagesBranch
refactor(Gruntfile): Tasks refactoring

Tasks have been changed:

`isGhPagesBranch` to `is-gh-pages-branch-exist`

`isClean` to `is-clean`