Added the error matcher.

Regression fix: let `have_id` work with nils.

Improved consistency on matcher arguments.

Improvements to `have_attribute` and `have_relationships`.

Minor fixes and support for meta subsets.

Support for documents with indifferent access.

Version 0.0.4

Release v0.0.3

Fixes:
 * Dependencies not loading/defined (rspec &co)
 * `have_link(s)` bug, added tests
 * Cleanups in docs and tests

Prepare for v0.0.2 release