Add Explanations and Custom Error Codes for Import Actions

Added descriptions for each non-standard API action in the Import
resource, and added custom Error Codes for the Create Action.
Added `from` and `to` as query parameters when getting imports

Consumers might want to filter their imports based on a date range, so
having filter parameters to support this would be useful.
Updated inputs for Import actions to follow new format

The new format for inputs in our api specification is:

field_name
	:*Required?* **field type**
	:description.

Since there were a lot of inputs in the Import action, updating their
format is it's own commit.
Remove `from` and `to` fields from the Invoice object

The `from` and `to` fields are only used for determining the entries
to associate with this invoice, so they aren't useful for consumers to
read.
Rewrote Invoice specifications to be easier to use & more descriptive

The invoice specification now follows our new format for parameter
specification, includes custom error codes, and explains what happens
with non-standard API actions. We also updated the specification of
our actions and objects to be easier to use and remove unnecessary
fields.
Rename `hashes` to `objects` because we are working in JSON

What Ruby considers `hashes`, JSON considers `objects`, so we needed
to update our terminology appropiately.