modified killworth variance est to not use weights; changes to vignettes

the killworth variance estimator's denominator no longer uses the survey
weights (which is most consistent with the original paper). also, slight
improvements to the vignette which introduces the scale-up estimator.
use respondent sum of degrees instead of estimated total
doc changes
doc changes
not ignoring vignettes any more
adding vignettes dir
vignette update
trying to get vignettes to work...
removing old-location vignettes
added ggplot2 as a req't (for vignettes)
changed vignette index entry

... it was wrong before; it had just been copied from a different
vignette. now it is right.
update scale-up vignette

filled in some TODOs, and also added an explanation of topcoding
and of attaching known populations to the survey data frame
small changes to helper functions

* add.kp now takes total.popn.size as an argument, in addition to
  a vector of known population totals
* topcode function now has an example to show in the help file
changes to documentation

minor changes to documentation files related to
some of the utility functions and the variance estimation routines
added require(functional)

this appears to be necessary because one of the functions is
defined using Curry from the functional library
changes to RDS variance estimation

refactored and rewrote both of the bootstrap routines that
are used with RDS samples
updates to vignettes

added two new vignettes, generalized_scaleup and rds, which
will have details on those two methods. also added some content to
network_scaleup, and some very minor changes to getting_started.
none of the vignettes is finished yet.
made text consistent with code

in some places, the text was not in sync with the code; it referred to
assuming that the known populations had been attached to the dataframe,
and to making estimates of proportions instead of totals. this is now
fixed.
more sketching out rds vignette

more work on first draft of the rds vignette; it is not yet finished
fixed call to Curry

needed to use functional::Curry to avoid explicitly putting a require()
at the top of the file. now it works with devtools.