Enabling snapshot plugins for spring-plugin-milestone

Fixing issue https://github.com/jhipster/generator-jhipster/issues/829 where a new machine without the RC2 version of the plugin cannot resolve the maven dependency.
fix(Grunt): package external css

add custom step to usemin:css flow to allow packaging of custom and  external CSS in one file while being able to run the autoprefixer task
Remove the user redefinition (cause compile error)

user is defined twice :
User user = userRepository.findOne(userDTO.getLogin());
User user = userService.createUserInformation(userDTO.getLogin(), [...]
The redefinition cause compile error on java 1.7
Merge pull request #834 from unknownboy/fix-for-java1.7

Remove the user redefinition (cause compile error)
Added javax.mail to logback
Corrected the error messages in the registration controller
The user PK is a technical ID, and not the login

Fix #807
Merge branch 'master' of github.com:jhipster/generator-jhipster
Make grunt hot reload .js files again

grunt didn't watch js files in the new modular frontend this fix makes js hot reload working again.
Comment was duplicated. A new comment about the ribbon was added
Merge pull request #838 from henri-tremblay/duplicate-browser-prompt

Comment was duplicated. A new comment about the ribbon was added
Save the changelog so we can overwrite it when the file is re-generated
Formatting
Updated IDs so they work on MySQL
Removed the hand-written query to use Spring Data instead
Removed the second hand-written query, to fully use Spring Data
Better usage of Spring Data for making the requests
Merge branch 'master' of github.com:jhipster/generator-jhipster
Merge pull request #839 from gomesp/issue-829

Enabling snapshot plugins for spring-plugin-milestone
Merge pull request #837 from atomfrede/grunt-js-watch

Make grunt hot reload .js files again