[SPARKR-94] Add a  method to get an element of a pair RDD object by key.
Merge pull request #82 from edwardt/master

Update readme for spark version 1.10 as merge commit 67d067a206e has upgraded to build against 1.1.0
Add {} to one-line functions and add a test case for lookup where no match is found.
Merge pull request #84 from sun-rui/SPARKR-94

[SPARKR-94] Add a  method to get an element of a pair RDD object by key.