Import packages from standard library to determine correct API URL
Skeleton of types for Post and parameters for requesting a blog's posts
Constants for post types specified by API, function for determining
correct post URL, skeleton of Posts liked by a blog as well as posts
posted in a blog
Be consistent with naming with the documentation
Add PostType to PostRequestParams
For blog methods we'll always have "blog" in the URL
The entityURL is always dependent on what blog we're asking about for
blog-related entities
Reversed type with variable
Build URL for requesting posts from a blog
Add custom error type
Add remaining parameters to PostRequestParams plus basic validation
Move PostRequestParams closer to method that uses it.  Extend
validation.  Add Filter parameter as specified by Tumblr API
Add reblog_info and notes_info to building query string
Take advantage of zero initialization (don't require pointers for nil
values).  Process remaining query string values.
Factor out Limit and Offset since they're used so frequently elsewhere
All the methods should include an error return value
Make generic structures for decoding API data.  Include net/http to
implement callAPI().
Followers and Likes can have their own file.
Factor out blog followers functionality into its own file
Factor out errors into their own file