Attempt to get budget info

[eileenmcnaughton]

@mattwire FYI - I am kicking the tyres on getting budget info out of Xero - not gone very far so far but am thinking the PULL would only 'get' unless a companion (civi_budget) extension is installed which would add the Budget entities / data storage

[mattwire]

@eileenmcnaughton Sounds good. My branch has a working "Items" API but haven't found the time to put in it's own PR.

Also, you'll want to change some variable names so they don't say "TrackingCategory" :-)

[mattwire]

Oh, and think I hit the same question with "Item" Pull. Currently it's just a very simple Get and I wasn't sure what return values should look like. I'm actually using the Create API in a custom client extension that creates/adds items to invoices before pushing to Xero. Their Create API is good in that it's actually a create/update API so you can call it if the item already exists. I think it makes sense for Xero extension to provide the APIs in a basic form that can be used for more clever things by other extensions but it might be nice to think about a reasonably standard format for "get" to return.