Add jre8 versions

Fixes #4

Much changes.
```diff
$ diff -Nru 8-jre7/Dockerfile 8-jre8/Dockerfile
--- 8-jre7/Dockerfile	2014-11-20 13:31:53.587697874 -0800
+++ 8-jre8/Dockerfile	2014-11-20 13:31:54.319665844 -0800
@@ -1,4 +1,4 @@
-FROM java:7-jre
+FROM java:8-jre
 
 # add our user and group first to make sure their IDs get assigned consistently, regardless of whatever dependencies get added
 RUN groupadd -r tomcat && useradd -r --create-home -g tomcat tomcat
```

It should have no compatibility problems according to the [apache tomcat site](https://tomcat.apache.org/whichversion.html).
> Each version of Tomcat is supported for any stable Java release that meets the requirements of the final column in the table above.