ModelMapper as optional dependency

[bmeurant]

ModelMapper is today a mandatory dependency but is not required by resthub. Should at least move to an optional dependency.

[bmeurant]

if needed, modelmapper should be explicitely added to project:

        <dependency>
            <groupId>org.modelmapper</groupId>
            <artifactId>modelmapper</artifactId>
            <version>0.6.3</version>
        </dependency>

        <dependency>
            <groupId>org.modelmapper.extensions</groupId>
            <artifactId>modelmapper-spring</artifactId>
            <version>0.6.3</version>
        </dependency>