Add sort param to PR's getAll method.
Add sort param to PR's getAll method.

trektech/test1,cbargren/github-xhr,blamattina/node-github,Phanatic/node-github,onesfreedom/node-github,kstarsinic/node-github,eistrati/node-github,codeschool/node-github,kaizensoze/github4,ludovicderochethales/node-github,apiaryio/node-github,jamestalmage/node-github,dfarr/node-github,Gerhut/node-github,ekristen/node-github,reviewninja/node-github,tempbottle/node-github,kaizensoze/node-github,meetearnest/node-github,AkashSC/MyRepoDOMO