Wrote API basics about File Uploading, updated Imports & Current User

Wrote an explanation in the API basics about how File uploads will
work, detailing how users must set their `Content-Type` headers
appropriately and send their data as HTTP key/value pairs. I also
updated the "create" action in the Import resource and the
"update profile picture" action in the Current User resource to link
back to API basic section.
Wrote first draft for how to archive multiple projects at once

To make the API more useful, we have included an additional action for
the Project resource that allows for multiple projects to be archived
in a single request.
Added support for merging multiple tags into a single current tag

Merging tags is an essential feature of the tag resource, since it
helps prevent too many tags from being generated. To help make the API
easier to use, we support merging multiple tags into a single tag at
once, since it is an often-requested feature.
Allow multiple Tags to be deleted at once

Because there can be *a lot* of tags, it will be helpful for consumers
if multiple tags could be deleted in a single API call.
Allow for multiple entries to be marked as invoiced outside of Freckle

To help make the API easier to use for consumers (particularly
consumers who previously or currently use a different invoicing system),
We have added support for multiple entries to be marked as invoiced
outside of Freckle.

This will also make it easier for external services that include invoicing to integrate with Freckle, since they don't have to make
multiple requests to the API in order to mark entries as invoiced.