Limiting access to R session from JavaScript

• Now most of the request from the server must be manually authorized in the R session to prevent misuse of publicly available
  apps based on jrc. Functions authorize, allowVariables, allowFunctions, limitStorage have been added. Check their man
  pages for more information.

• openPage now have browser argument, which allows to specify a browser to open a page (previously the default browser was
  used with no alternatives).

• Function getPage is added. This function returns the main page-handling object with all the information about current session.

• Function callFunction added on both R and JavaScript sides. It allows to call a function by name, list of arguments and name
  of variable to which assign the result.

• Now port for the local server is selected by httpuv::randomPort() function. User can also provide a port number as an
  argument to the openPage function.

• sendData now has argument rowwise which allows to send matrices and data.frames to JavaScript not only rowwise (default),
  but also columnwise.

• If in sendData keepAsVector = FALSE jrc now checks recursively for any arrays of length 1 to replace them with scalars
  (important for lists).

• Some bugs with changing variable types in jrc.sendData fixed.