Add MapQuest support

[Diego H.]

Following #62 , I have implemented MapQuest:

• Docs: https://developer.mapquest.com/documentation/geocoding-api/

All test passed, including:

• External tests
• testthat
• devtools::check()
• Tests on cran-comments

Note that batch geocoding has a 100 query limit. My overall impression is that is a fast geocoder, but not specially accurate on locations outside USA on forward geocoding.