Update readme.md

the url has changed to https://www.pushbullet.com/account
Merge pull request #3 from QuantumGhost/master

Change old style class to new style class
Merge pull request #4 from chid/patch-1

Update readme.md
Update readme.md

(Sorry about split update, anyway pushbullet was recently released for iOS)
minor fix mentioned in #2
Merge pull request #5 from chid/patch-1

Update readme.md