Add href locations to html_dependency

htmltools will need to be used in contexts where dependencies exist
not on disk, but at certain URLs. This commit introduces explicit
href locations to HTML dependencies.

Also these minor changes:

- html_dependencies_as_character now uses dependency name and version
  as subdirectory name when copying to lib dir
- Perform URL escaping on dependency filenames
Initial dump of Shiny tag library
Heavy refactoring

- Change function names to camelCase for consistency
- htmlDependency: Normalize "src" value at creation time and store as list
- attachDependency: Put single dependency argument in list
- Split html_dependencies_as_character into separate functions: copyDependencyToDir and renderDependencies
Version bump
Add as.tags S3 method to allow arbitrary objects to be embedded in tags
Customize renderDependency path building behavior
Add HTML preservation functions
Fix broken htmlPreserve, and document
Merge pull request #5 from rstudio/0.2

Major refactor and addition of tags functionality from Shiny