refactor to use httr to handle requests
more tests for httr GETs
let httr handle cleaning up search terms
option for query-builder to return httr response object
Pass on httr options to the url maker
httr integrated
README bugs
httr examples given in README
updates to docs, bump version to reflect httr swap

These changes, combined with the last several committs mean httr is
integrated into rentrez in functions and documentation. So others can
use this functionality this version will be the basis of the next CRAN
release, bumped up to version 0.3.

This also closes #17