Add config option to ignore GEM_HOME

Previous behavior was to respect GEM_HOME at runtime, so that gems
in GEM_HOME would be loaded in preference to versions packaged in
the jar file. With this commit, that behavior is still the default.
However, by setting

    config.ignore_gem_home = true

the value of GEM_HOME is not respected at runtime. This means that
gems will ONLY be loaded from within the jar file.
Merge branch 'jar-paths' of github.com:wasnotrice/warbler into wasnotrice-jar-paths

Conflicts:
	lib/warbler/config.rb
	lib/warbler/templates/jar.erb
renamed ignore_gem_home/allow_gem_home_override to override_gem_home