Add metadata

[lindsayplatt]

Now each gsplot object will have a top-level list called metadata. This PR adds 2 fields: created which is the date the object is/was created and gsplot.version the gsplot version used to create the object. Both can be manually input as arguments to gsplot().

This PR also adds to access functions to get those metadata values - whatDate and whatVersion.

See #304

[coveralls]

Coverage increased (+0.09%) to 82.226% when pulling d0d1e7f on lindsaycarr:addMetadata into e839398 on USGS-R:master.

[jiwalker-usgs]

I like this, I'm wondering if there is other metadata we should consider. Also, are there other accessor methods that could follow the what pattern here?

[jiwalker-usgs]

might use packageVersion("gsplot") here instead

[jordansread]

better yet is

packageVersion(getPackageName())

so we don't need to modify it if we change the name of the package.

[lindsayplatt]

I tried packageVersion("gsplot") at first, but it returns a list instead of a character string. packageDescription("gsplot", fields = "Version") returns a single string. To get the output of packageVersion into a single string seemed more complex than using packageDescription.

I will make the package reference generic per @jread-usgs's suggestion.

[ldecicco-USGS]

What is this magic "Approved" that you guys are doing?

[ldecicco-USGS]

Ahhh! Cool.

[lindsayplatt]

@jiwalker-usgs I think there were other metadata that people suggested, but we decided that #304 would just be the gsplot version and date.

[coveralls]

Coverage increased (+0.09%) to 82.226% when pulling 2327280 on lindsaycarr:addMetadata into e839398 on USGS-R:master.

[lindsayplatt]

@jread-usgs the appveyor build failed...is that OK?