Gradle build

In this pull request I have removed all IntelliJ related files and instead replaced them with a gradle build file. The gradle build file is able to generate the lexer + grammar, build the plugin and run the plugin in idea / debug it. It can also run the tests. We should also be able to use this build script on a CI server.

Removing IntelliJ related files should hopefully stop annoying popups about changed *.xml files in the idea directory when updating. You should still be able to create an IntelliJ project by importing the gradle.build. It may be possible in the future to specifically generate a 'plugin' intelliJ project from the gradle files, but I have not yet had a chance to look at.
Should this be `gradle.properties`?