Decouple the importer class from WP-CLI

The `Importer` class was decoupled from WP-CLI except for its use as a
progress/error reporting function. I’ve fixed that by abstracting the
message reporting into two new methods, and implementing the use of
WP-CLI for reporting in a child class. We then use the child class for
importing.

See #82
Move the import function to the importer

See #82
Remove errant compact()

See #82
Use $this instead of $importer in importer class

See #82
Load WordPress during unit tests
Add a parent test case for the import tests
Have the export test case extend WP_UnitTestCase
Add initial tests for importing

Covers basic importing of a file and a function.
Merge pull request #131 from JDGrimes/issue-82

Decouple the importer from WP-CLI