fuzzy file searching within project

NOTE: Not ready for merge, but ready for discussion. This work supersedes #235 by integrating the same functionality into the existing completion popup display.

![file-search](https://cloud.githubusercontent.com/assets/1976582/5273923/c9006466-7a42-11e4-9bc0-997b50ac2099.gif)
 
To make this efficient, we re-work the completion display a little bit:

1. We display a maximum of 100 completions in the UI popup, and display a `Completion list is truncated` message below if there are too many to display, to ensure a fast UI response.

2. If there are too many completions (> 2000), then we do not even bother sorting the list, as this will be too slow. However, when it is not too large we use a priority queue to obtain a partial sort of the top 100 items.

There is still work to be done in handling the situation where the number of files is too large to produce completions for the whole subtree of a path; likely through using `listFiles`.

As an aside -- should we consider distinguishing files and directories in the popup icon?
It looks like .rs.getProjectDirectory can return R_NilValue. If this happens then I think an error might occur in the call to .rs.normalizePath -- is that correct?