Improve configurability in Pippo via DI

[decebals]

For example now, I have in Application class methods as:

• setTemplateEngine
• setRouter

My idea is to add as dependency the tiny inject-api library in pom.xml (probably with scope provided):

<!-- https://mvnrepository.com/artifact/javax.inject/javax.inject -->
<dependency>
    <groupId>javax.inject</groupId>
    <artifactId>javax.inject</artifactId>
    <version>1</version>
</dependency>

and add @Inject annotation on all setter methods that are possible extensions points in Pippo.

In this mode we can configure completely Pippo using a DI container as Spring Framework or Guice.