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.
added draft test files
Mark all items in a deprecated file as deprecated
Use the function call reflection to determine the version
modified comment indentation and file names
Merge pull request #132 from lucatume/master

added draft test files
Merge pull request #133 from atimmer/deprecated-files

Mark all items in a deprecated file as deprecated
Merge pull request #131 from JDGrimes/issue-82

Decouple the importer from WP-CLI
Prevent an error when parsing test files

When parsing test files there was an error that $m[3] didn't exist, check for that. $m[1] will always exist even though it is optional because of PHP's handling of preg_match.

Also improves clarity for the next reader.
Merge pull request #134 from atimmer/fix-error-and-clarity

Prevent an error when parsing test files
Add deprecated file to test for that functionality
Merge pull request #135 from atimmer/deprecated-file

Add deprecated file to test for that functionality