Needed for Java9

Stop swimming against the tide and accept what most of the rest of
the world does.

While it adds an annoying depth to the actual source code, it makes
the root directory nicer (no more 'test-res' dirs), means we don't
have to override the gradle config, and makes it nicer to work in
IDEA 2016.1.

Issue #3

This replaces the default source directories (src/main/java, etc)
with our own, instead of just appending ours to the list.

Use @Nonnull

Change-Id: I3b054eb2144f51bc0b5935ee24525616dc042a4e
Reviewed-on: http://gerrit.dmdirc.com/4047
Reviewed-by: Chris Smith <chris@dmdirc.com>
Automatic-Compile: DMDirc Build Manager

Change-Id: Iea36563c1cd6119e3da0b2d200ceb95d09092fd8
Reviewed-on: http://gerrit.dmdirc.com/4005
Reviewed-by: Greg Holmes <greg@dmdirc.com>
Automatic-Compile: DMDirc Build Manager

Tidy up .gitignore.

Change-Id: Iaab43f6e475d834644a5c18a983a24679874df52
Reviewed-on: http://gerrit.dmdirc.com/3913
Reviewed-by: Greg Holmes <greg@dmdirc.com>
Automatic-Compile: DMDirc Build Manager

Change-Id: I579afbde086d5761fe3743b23f4655cb4ea5b8df
Reviewed-on: http://gerrit.dmdirc.com/3909
Reviewed-by: Greg Holmes <greg@dmdirc.com>
Automatic-Compile: DMDirc Build Manager