adding a new function fieldSearch

This pull request looks a little bigger than it is:
* `src/fieldSearch.cpp` is the main piece, and it is under 100 lines of code. I used my preference for indenting, and using a namespace alias -- but this is _your_ project so by all means feel free to re-indent and change.
* I used Rcpp Attributes for this.  If you work on the shell (rather than in RStudio which does it for you), then you need to call `Rcpp::compileAttributes(".")` each time the interface changes.  And only then. Normal code changes are fine.
* I also used roxygen2 to write a little bit of documentation. I usually call that via a littler script doing just the roclets (as I dislike roxygen messing with NAMESPACE and DESCRIPTION) -- a call is eg `library(roxygen2); roxygenize(".", roclets="rd")`.  Again, RStudio makes that a little easier.
* The other files are just varnish related to having RStudio around, or side-effect from using `compileAttributes()` and `roxygenize()`.

Shoot me questions here if you have any.