Improve template usability and fix errors

[CreeperPookie]

Although this template has saved me so much time compared to dealing with the official Forge MDK, something I immediately noticed was that it didn't work right off the bat - there were a lot of weird errors that weren't immediately obvious why were happening; So, I decided to fix them all :D
Specifically:

• IntelliJ doesn't actually support Gradle 4.4.1, so I bumped that to Gradle 4.5
• the :jar task in buildscript/forge-1.7-mixin.gradle doesn't actually exclude files correctly from the embedded META-INF folder; this becomes especially apparent when Mixin is enabled, as it causes Java to refuse to load the jar
• The default mcmod.info actually refers to minecraft_version, not mcversion, so trying to expand mcversion really doesn't make any sense
• All the classes are relying on Gradle inserting packages in at compile time; not only would this work poorly in IDEs (they would show a lingering error at best, and parse the file extremely wrong at worst), but Gradle doesn't even support this (as far as I can tell anyway); not only that, but attempting to gradlew build causes a large amount of errors, due to the effectively invalid syntax
• The ExampleMod class is trying to use an inserted package (as covered), but ExampleMod.java wasn't even in any package; even if it was expanded, it would evaluate to package ;, which is obviously invalid
• The filenames of the Java files don't correspond to standard naming, as wouldn't be possible to name a class with a . (as in public class foo.bar {} is not valid syntax)

[CreeperPookie]

was trying to rename my branch locally, forgot that would effect the PR
sorry about that lol
GitHub keeps breaking lol