Shim help, ?, and system.file on package load

These functions are now inserted into a parent environment of global
when a package is loaded, instead of existing in the devtools package
environment. This is so that the functions are available when a package
is loaded, even if devtools itself is not attached.
Documentation improvements
Update NEWS
Fix shim tests
Don't lock bindings for shims
Fix usage section for help docs
Merge pull request #464 from wch/help-shim

Shim help, ?, and system.file on package load