Wrap "impl" into an array in build require call

Require call was different between "build" and "runtime" evaluation: During runtime the implementation was wrapped into an array, during build it was not.
Using the same implementation map for both cases did not work.
Fix property name in impl map doc

Fixes #12
Merge pull request #11 from lbernau/master

Wrap "impl" into an array in build require call
under-caffeinated