Don't modify arguments in syncer

Definitely not good practice to modify arguments directly. Better to clone them before interacting with them.
Update tests to reflect cloned options
Oops, forgot to remove one argument in the test.
Don't need to change argument name.
add test for getBootstrappedData
add ability to clear cached layouts
add test for passing through the error object
add test for the layout template cache
Update examples to underscore@~1.5.2

To match rendr's dependency on underscore@1.5.2.
Merge pull request #314 from airbnb/update-underscore-in-examples

Update examples to underscore@~1.5.2
Merge pull request #311 from lusini/improve-viewEngine-tests-2

Improve view engine tests
Merge pull request #309 from jeromegn/patch-2

Don't modify arguments in syncer