Add more information to README.md

Add a link to the wiki page and some precompiled build information to
the README.md file.
Generalize JDK download link in README.md

The JDK download link, which is located in the README.md file, now
points to an overview page instead of a download page for a specific JDK
version.
Improve performance of DFH.get() and function invocations

* Improve the performance of DefaultFeatureHolder.get() by leaving out a
validation statement.
* Improve the performance of DefaultFunction.invoke() by improving the
argument validation process.
Move function executor sorting to DefaultFunction for better performance

Move the sorting of FunctionExecutors by priority form
DefaultFunctionInvocation to DefaultFunction for increasing the
performance when invoking a function. That removes the need for the
FunctionExecutorContext class.
Implement hard-coded hashCode()/equals() methods to improve performance

Implement hashCode()/equals() methods without external utilities (like
reflection builders) in critical classes (like DefaultFeatureHolder) in
order to improve the overall performance.