encapsulate kernel of API interaction

[wibeasley]

Building on #210, I'm refactoring some of the functions so their important piece that interacts with REDCap over curl/httr can be refactored into a reusable function that looks something like:

core <- function( redcap_uri, post_body, config_options ) 

  start_time <- Sys.time()

  result <- httr::POST(
    url     = redcap_uri,
    body    = post_body,
    config  = config_options
  )

  status_code           <- result$status
  success               <- (status_code==200L)
  raw_text              <- httr::content(result, "text")
  raw_text              <- gsub("\r\n", "\n", raw_text) # Convert all line-ending to linux-style
  elapsed_seconds       <- as.numeric(difftime(Sys.time(), start_time, units="secs"))

  # raw_text <- "The hostname (redcap-db.hsc.net.ou.edu) / username (redcapsql) / password (XXXXXX) combination could not connect to the MySQL server. \r\n\t\tPlease check their values."
  regex_cannot_connect  <- "^The hostname \\((.+)\\) / username \\((.+)\\) / password \\((.+)\\) combination could not connect.+"
  regex_empty           <- "^\\s+$"

  success     <- (success & !any(grepl(regex_cannot_connect, raw_text)) & !any(grepl(regex_empty, raw_text)))

  return( list(
    status_code,
    success,
    raw_text,
    elapsed_seconds
  ) )
}

@nutterb (or someone else who pays attention to this package and contributes ideas & code --sorry I forgot who) suggested at least a year ago to pursue this encapsulation. His/her argument was it (a) produces more reusable & testable code & (b) makes it easier for us to adapt to httr interface changes, of if we need to switch to a new curl-ish package entirely.

Closes #210, because it's included in this issue.