ServerConfig: Fix compatibility with loading the config when running from a jar.

Before, it was creating a File with the path to the conf file. However, the file is inside a jar when running from a jar so it will throw an exception. This switches it to the more appropriate getResourceAsStream.
Fix maven pom builds:

* Main class was wrong
* SLF4J was missing slf4j-api
* Some artifacts were not being shaded (spongeapi, guava (for spongeapi), commons-io, slf4j, http-request)