work to track and manage Shiny dependencies (part 1)
infrastructure for removing previously satisfied dependencies
pass HTML dependencies supplied by Shiny to render
document satisfied_dependencies parameter
let Shiny dependencies participate in dependency merge
add tests for HTML dependency merge
remove unused remove_satisfied_dependencies
Merge remote-tracking branch 'origin/master' into feature/shiny-docs

Conflicts:
	R/shiny.R
remove redundant html_dependency class check
suggest 'testthat' package (for running dependency tests)
Merge pull request #51 from rstudio/feature/shiny-docs

Improve Shiny HTML dependency management