[FEATURE RELEASE] Added support for generic template engines

#Template engines support
The project is now organized in modules. The sender service along with the scheduler are in the following dependency in Maven

<dependency>
    <groupId>it.ozimov</groupId>
    <artifactId>spring-boot-email-core</artifactId>
    <version>0.3.0</version>
</dependency>

To support template engines, specific modules must be added.
To embed the module that includes the Freemarker template engine, you can use the following Maven dependency:

<dependency>
    <groupId>it.ozimov</groupId>
    <artifactId>spring-boot-freemarker-email</artifactId>
    <version>0.3.6</version>
</dependency>

for Mustache:

<dependency>
    <groupId>it.ozimov</groupId>
    <artifactId>spring-boot-mustache-email</artifactId>
    <version>0.3.6</version>
</dependency>

for Pebble:

<dependency>
    <groupId>it.ozimov</groupId>
    <artifactId>spring-boot-pebble-email</artifactId>
    <version>0.3.6</version>
</dependency>