fixes #456: use .Random.seed only if it exists in the global environment

e.g. the expr may not trigger the creation of .Random.seed, such as set.seed(NULL) under R 2.15.x, which will fail
introduce an equivalent function to achieve set.seed(NULL) in R 2.15.x
Merge pull request #499 from yihui/bugfix/456

Bugfix/456
use @export instead of @S3method
htmltools was in both Depends and Imports, against which R CMD check will warn

I guess it was in Depends because otherwise as.tags could not be treated as an S3 generic; sounds like a bug of roxygen2 or devtools
cosmetic changes after #481, and roxygenize
no need to assign a copy of these functions in shiny, because we have imported htmltools, and we only need to export the imported functions
bump version