Get rid of warnings in Spoon's build

I think it's about time we clean up the compile output and actually start to care about warnings, or even better, treat warnings as errors and selectively suppress the ones that we don't care about.

I've been bothered by the warnings in the build output for a good while. All of the garbage warnings obscure "real" warnings, such as deprecations in libraries we depend upon.

I propose two things:

1. We get rid of the current warnings.
2. We treat warnings as errors so we can keep the build output clean, and actually act upon new warnings.

To get the warnings:

```
mvn clean compile
```