Add support for the LineItem resource and APIs

[remi-stripe]

r? @richardm-stripe
cc @stripe/api-libraries

[remi-stripe]

golint -set_exit_status ./...
lineitem/client.go:9:6: type name will be used as lineitem.LineItemIter by other packages, and that stutters; consider calling this Iter
Found 1 lint suggestions; failing.
make: *** [lint] Error 1

@brandur-stripe Any chance you could have a look at this PR tomorrow morning? I'm struggling to understand how to represent the LineItem resource's ability to be paginated. We don't really have a pattern line this today in stripe-go and I'm not sure what the right way to approach this is.

[brandur-stripe]

@remi-stripe Hey! Just taking a look at some of our existing conventions, it looks like we usually call the "main" iterator in any particular client package just Iter usually, with any "secondary" iterators getting an object prefix. For example:

• In creditnote, the credit note iterator is Iter while the line item iterator is LineItemIter.
• In invoice, the invoice iterator is Iter while the line iterator is LineIter.

I imagine for this one you just ended up copying the LineItemIter from creditnote — would suggest renaming it to just Iter.

[richardm-stripe]

LGTM