README.md: remove obsolete tasks
POM: bump to pom-scijava 5.0

This also makes some minor improvements to the POM.
RScriptLanguage: implement the decode method
RTest: remove erroneous comment

It was copy/pasted from elsewhere.
RTest: add another unit test

This makes the testBasic test more in line with the other script
languages, and converts the prior testBasic test into its own unique
R-specific test.
Start implementing the RBindings properly

It is not finished, and does not yet work. But at least it begins with
the idea that the bindings must be backed by the RConnection now,
instead of using a bogus map structure which would never work in practice.
RScriptEngine: remove unused helper methods
RUtils: simplify getVar implementation

The hope is that the asNativeJavaObject() method is
good enough to return the right thing in all cases...