Feature/fuzzy finder improvements

A number of improvements to the fuzzy file finder (re: @hadley's comments)

1. Exact matches now receive no penalty (so we can be sure that the query `build` will place a function `build` at the top)

2. `json`, `yaml` and `yml` files are now indexed (although the `.yaml` mode highlighting is kind of ugly / not that useful)

3. The scores for files are penalized a bit (so that functions with equal scores are preferentially chosen)

Some other changes rolled into this one:

1. Tokenization for strings is fixed in R modes (it turns out that e.g. `"abc"` was tokenized into two pieces; as `['"', 'abc"']`!)
2. The scope tree in R modes now displayed the function name as well as the function arguments (I believe this will be a popular change; now one can see what arguments a function takes while they work within it)

![screen shot 2014-10-28 at 12 59 41 pm](https://cloud.githubusercontent.com/assets/1976582/4816139/004b3b2a-5edd-11e4-8449-4213b2c6595d.png)

![screen shot 2014-10-28 at 1 00 07 pm](https://cloud.githubusercontent.com/assets/1976582/4816144/0b843550-5edd-11e4-9eda-1bea729d9161.png)

If you exclude the second argument to substr it just means "all the rest of the character in the string" so you should be able to just pass lastDotIndex by itself. Note also that we've traditionally included the "." in the extension so you might want to update the "rd" test to be ".rd" (and have this function return the extension including the dot)