Mark `print` as exported
Simplify installation

Fully qualify `install_github` installation so that the code works out-of-the-box, without having to load `devtools` first.
Only import operators if there are any
Simplify module and operator attachment code
Patch .Rprofile
Add test cases for detach on unload
Detach modules when unloading them
Fix redundant unload in old test
Reattach modules correctly when reloading
Merge branch 'detach_on_unload' into develop

Resolves #27.
Complete change notes
Fix bug in `import` documentation
Update README to reflect recent semantic changes
Add change notes and new version numbers
Test that circular imports work
Implement circular imports

Ensure that unfinished module namespaces are made known internally
before their source file is loaded, in order for incomplete imports due
to circular references to be found, rather than re-loaded.
Merge branch 'issue-17' into develop

Fix #17