Auto creation of authenticated users

* Abstralize existing authethication interceptor
* Create default authentication interceptor using userService (old implementation)
* Extend this interceptor to allow users' auto creation
* Write some tests for this functionalities

Those functionalities can be activated with a property overrider with the properties:

```
geostoreAuthInterceptor.autoCreateUsers=true
geostoreAuthInterceptor.newUsersRole.role=USER
geostoreAuthInterceptor.newUsersPassword=NONE|USERNAME|FROMHEADER
geostoreAuthInterceptor.newUsersPasswordHeader=newUserPassword
```

Some documentation for this options:

* **autoCreateUsers**: Flag to indicate if an user that not exists could be created when it's used
* **newUsersRole**: New role for the user (default it's USER)
* **newUsersPassword**: New password strategy. Can be: NONE --> empty password, USERNAME --> password = username, FROMHEADER --> see newUsersPasswordHeader
* **newUsersPasswordHeader**: Header key for the new password if the selected strategy is FROMHEADER
Basic authentication interceptor

* New authentication interceptor without user autocreation
* Test for this authentication interceptor
Rename authentication interceptors

* Rename authentication interceptors to mantain retrocompatibility.
* Configure the old implementation as default interceptor
* Include the new interceptor commented for /users services
* Change the properties overrider for the new interceptor
Merge pull request #47 from alediator/issue_43

Auto creation of authenticated users