adds .gitignore
adds mac dirs for jdk and idea to build.properties
adds build.properties to gitignore
backs out changes that are incompatible with JDK 1.6.

JetBrains only officially supports JDK 1.6 on OS X, see
https://intellij-support.jetbrains.com/entries/23455956-Selecting-the-JDK-version-the-IDE-will-run-under
To maintain cross platform compatibility, plugins should not be built
against JDK 7.
adds license file
fixes misnamed xml name in plugin.xml
added better error reporting for missing compiler

Previous behavior says that main is missing for a missing compiler
error, confusing the end user.