Fix to enable building with LESS 1.4

Since LESS 1.4.0 (~"@var") is removed.
We should use @{var}
Let's use Grunt instead of shell scripts

Available grunt tasks
* grunt css:debug
  Build CSS files from LESS files, without compression
* grunt css:release
  Build CSS files from LESS files, with compression
* grunt css
  same as "grunt css:release"
* grunt js:debug
  Build JavaScript files, SourceMap files and link them together
* grunt js:release
  Build only JavaScript files, without SourceMap files
* grunt js
  Same as "grunt js:release"
* grunt debug
  Same as "grunt js:debug css:debug"
* grunt release
  Same as "grunt js:release css:release"
* grunt
  Same as "grunt release"
Fix minor problem in README
Fix to enable building with LESS 1.4

See https://github.com/twbs/bootstrap/pull/6780
State that build instructions are in build folder
Merge branch 'patch-1' of https://github.com/mlocati/concrete5 into grunt_merge
Add package.json

Add package.json for automated dependecy installation
Update instructions because of package.json

Now it's enough to do 'npm install' do install the dependencies.
Merge pull request #1 from mlocati/grunt_merge-readme

Update instructions because of package.json
Update Build README for npm install specifics

Update Build README for npm install specifics
Merge pull request #1397 from mkly/grunt_merge

Grunt merge