Support draft 4 'dependencies'

[oatkiller]

Hello, Thanks for all of the work on jsl. I am curious if jsl supports the 'dependencies' keyword as documented here:

https://github.com/json-schema/json-schema/wiki/Dependencies and https://tools.ietf.org/html/draft-fge-json-schema-validation-00#section-5.4.5

If it is not, I am curious about adding support. Thanks!

[oatkiller]

Here is an example of usage: http://stackoverflow.com/a/40293011/255345 Seems cool!