this changes tests to only use the dev client as all code is shared
between that and the write.as client, just the host changes.

also updated post tests to round trip and test over sub tests

tests were failing on expected write.as blog title
previously the title was 'write.as' and it is now 'write.as blog'

also removed test case ExampleClient_GetCollection as was a duplicate of
TestGetCollection

this adds a client method to retrieve a collection post along with basic
tests - phabricator task T588

This reverts commit 1d34eede72.

Rather than accepting a naked string, accept a DeleteCollectionParams
struct so that new optional parameters can be added in the future
without breaking the API.

This adds support for deleting collections.

Resolves #1.

This closes #3