Add sfc method and remove uneeded dependencies

[JosiahParry]

Hi @joelgombin,

Just discoverd this great interface! I've gone ahead and made a pull request that implements a method for sfc objects in addition to sf objects.

This pull request:

• implements sfc method
• modifies sf method to no longer use magrittr and dplyr
• removes magrittr and dplyr as dependencies
• updates test to check input and ouput class for sfc objects
• adds additional test to check that outputs are identical for all methods

[joelgombin]

Thanks @JosiahParry! Merged.

[JosiahParry]

Woooh! Fun :)