issue #393, issue #128

Added a REST API for user management
The REST API exposes the functionality available in module "users" for HTTP access.

The following methods are available:
* GET /_api/user/username: fetch user information
* POST /_api/user: create new user
* PUT /_api/user: replace an existing user
* PATCH /_api/user: (partially) update an existing user
* DELETE /_api/user: remove an existing user

Additionally, a module "crypto" is introduced and exposed for Javascript actions.
The crypto module provides the following functions:
* require("org/arangodb/crypto").md5();
* require("org/arangodb/crypto").sha256();
* require("org/arangodb/crypto").rand();
updated docs for user API
Merge branch 'devel' of https://github.com/triAGENS/ArangoDB into devel