Fix 'Error in Context$new : could not find function "loadMethod"'

This warning was happening to dependent packages on R CMD check.

The problem is due to delayedAssign; it appears this can't be used safely, at least not to define package-level symbols that contain S4 or reference class objects.

If you call this in a package's .R file:

`delayedAssign("hello", stop("boom"))`

but don't refer to "hello" anywhere, when you run R CMD check on a dependent package you'll see the error.

If the expression needs the methods package (like Context$new()), you'll get an error unless the dependent package itself depends on methods.
Merge pull request #421 from jcheng5/bugfix/context-new

Fix 'Error in Context$new : could not find function "loadMethod"'