Add "BOM" (Bill Of Material) for easier use with Maven

Guice extensions must generally have their version match the one of Guice. Maven provides an easy way to make sure this is the case with “BOMs” that you later import using `<dependencyManagement>`, `<type>pom</type>` and `<scope>import</scope>`: https://maven.apache.org/guides/introduction/introduction-to-dependency-mechanism.html#Importing_Dependencies

Use cases:

1. GIN 2.1.2 depends on Guice 3.0 and uses assistedinject; if you want to use GIN 2.1.2 and Guice 4, you have to explicitly declare a dependency (or _dependency management_) on `guice-assistedinject` even if you don't use it, otherwise you'll have Guice 4 and guice-assistedinject 3.0 in your classpath and it won't work.

2. Closure Templates also uses Guice 3.0 and uses assistedinject and multibindings.

By just importing a BOM you'd make sure you use the same version of all your Guice dependencies, either direct or transitive.