Elevatr

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Bug Fixes
- Primary change with this released is fixing a bug with the return file type on the AWS and mapzen APIs. "tif" was changed to "tiff" and the check was stopping processing of the raster images. Details are on <https://github.com/jhollist/elevatr/issues/17>. Thanks to the following individuals for catching this: yipcma, TomBor, jslingsby. And thanks to vividbot for <https://github.com/jhollist/elevatr/pull/18> which provided a fix.
- Thanks to pascalfust for kicking me into gear to send fix to CRAN.
- Fixed NOTE on CRAN: Packages in Imports, not imported.
- Removed prettyunits
- moved rgdal to suggests
- Changed where ratelimitr getting called (was not in a function so couldn't be exported/called.
- Fixed travis build errors caused by change in elevation API that now requires a key.
- Added deprecation message to get_elev_point and get_elev_raster, due to pending shutdown of Mapzen :(

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Bug Fix
- There was a typo in building the mapzen api key. Was masked prior as a keyless access was allowed. It no longer is and get_elev_raster was failing. That has been fixed
- Tests also failing due to keyless access. Encripted key now pushed for use on travis. Tests not run on CRAN
- Thanks to hrbrmstr for pointing me in the right direction on fixing the testing with an api key.
- Also thanks to noamross and ropensci for maintaing <https://discuss.ropensci.org> where I found <https://discuss.ropensci.org/t/test-api-wrapping-r-packages-with-oauth-tokens/157>. And thanks to jennybc for wrapping all this up and provide great guidance on testing and vignettes that require a key. That info is here: <https://rawgit.com/jennybc/googlesheets/master/vignettes/managing-auth-tokens.html#encrypting-tokens-for-hosted-continuous-integration>

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Minor Changes
- inst/doc was inadvertently included in package. This verisons removes that and includes only vignettes.

Added
- Core features for fetching and processing elevation data for multiple resolutions.
- Raster class for handling raster data (show, save, convert to numpy array).
- Basic documentation and examples.