Add support for the USA National Phenology Network (USA-NPN)

[amotl]

Hi there,

rnpn implements a client for the USA-NPN Data Web Service API by @usa-npn written in R. The API documentation is available at [1] and some example REST calls would be [2,3,4].

It would be nice to add this functionality also to phenodata, see also [5].

With kind regards,
Andreas.

[1] https://docs.google.com/document/d/1yNjupricKOAXn6tY1sI7-EwkcfwdGUZ7lxYv7fcPjO8
[2] https://www.usanpn.org/npn_portal/observations/getAllObservationsForSpecies.json?species_id[0]=52&species_id[1]=53&start_date=2008-01-01&end_date=2011-12-31
[3] http://www.usanpn.org/npn_portal/observations/getObservations.json?start_date=2012-01-01&end_date=2012-01-03&state[0]=AZ&state[1]=IL&request_src=rest_test
[4] http://www.usanpn.org/npn_portal/observations/getSummarizedData.json?start_date=2012-01-01&end_date=2012-01-10&request_src=rest_test
[5] https://github.com/hiveeyes/phenodata/blob/0.9.1/doc/usa-npn.rst