Satisfy BDR for CRAN Submission

[ramhiser]

Welp. Got Ripley'd over last night's CRAN submission. First, rather than ignoring the extended description in cran-comments.md, which remained the same, Ripley said:

None of this is relevant to an update of an existing package!

Next, because itertools2 is in the package title in DESCRIPTION, Ripley said:

Please do not repeat the name, and use title case as per 'Writing R Extensions'.

In case I didn't understand fully, Ripley closed the email with:

Do improve the title before re-submission, and reduce our reading load to relevant material.

[ramhiser]

In his R Packages book Hadley Wickham recommends:

I recommend that you store your submission comments in a file called cran-comments.md. cran-comments.md should be checked into git (so you can track it over time), and listed in .Rbuildignore (so it’s not included in the package).

He also gives an excerpt from a recent version of httr:

## Test environments
* local OS X install, R 3.1.2
* ubuntu 12.04 (on travis-ci), R 3.1.2
* win-builder (devel and release)

## R CMD check results
There were no ERRORs or WARNINGs. 

There was 1 NOTE:

* checking dependencies in R code ... NOTE
  Namespace in Imports field not imported from: 'R6'

  R6 is a build-time dependency.

## Downstream dependencies
I have also run R CMD check on downstream dependencies of httr 
(https://github.com/wch/checkresults/blob/master/httr/r-release). 
All packages that I could install passed except:

* Ecoengine: this appears to be a failure related to config on 
  that machine. I couldn't reproduce it locally, and it doesn't 
  seem to be related to changes in httr (the same problem exists 
  with httr 0.4).

[ramhiser]

Before updating the CRAN comments, I tested the package on a Rackspace instance running Ubuntu 15.04 (vivid). All tests passed without a problem.