Updated the set function

It now properly checks wether virtuals exist at all, instead of assuming they do.
Merge pull request #169 from johanneslumpe/patch-7

Updated the set function
Fixed get function

Same error as for the set function
Added test for get/set issue without virtuals
Merge pull request #171 from johanneslumpe/patch-8

Getter fix +  tests
A few tweaks to the virtuals plugin

Using the constructor rather than initialize
Calling Bookshelf::get rather than Backbone's
Fixes underscore methods and reduces code duplication.

Fixes #170. 

There is now a central (currently private) method for getting the resolved virtuals of a model.
Use proto instead of the long name, like the rest of the methods
Added tests for underscore methods with virtuals
Merge pull request #173 from nathggns/patch-4

Fixes underscore methods and reduces code duplication.
Bugfix on virtuals options, minor code formatting/cleanup