move ffindStartOfEvaluationContext to javascript; use it to get function names
avoid duplicates for scoped variable completions
remove debug output; autocomplete in more contexts
include other completions for 'attr', 'getOption' etc
autocomplete after 3 chars; backspace continues autocompletion
more autocompletion tweaks

- Get new completions on insertion of a '/' (for directories)
- Get new completions on insertion of '(' (for functions)
- Allow dashes in selections (mainly for e.g. ?"package-foo" style
  completions)
only show error popup if we can auto accept
roll back auto-popup on 5 char (too disruptive for interactive R use)
fix help popup; be more strict on paren insertion
tweaks for help
make sure we pass on the cached function name
invalidate pending requests here as well
ensure e.g. 'debug(stats::rnorm)' doesn't auto-insert parens
cleaner impl for getAutocompletionContext
don't provide completions if there is no token and no context
fix thinko re: grabbing function name
supply token type for findOpeningBracket works
Merge pull request #193 from rstudio/feature/r-autocompletion-tweaks

Tweaks for the new R autocompletion