Better error message for update()

Improves the error message for update() when the existing value is the same as the new one, as suggested in #1254
Better error message for update()

Return error messages for both identical values and update failures.
Don't throw error, as this will change existing behavior

More verbose tests
add test scenario for exporting posts from a given category
category information should be included before posts on export file
author_email is not always in the WXR file
Merge pull request #1570 from rodrigoprimo/import-author-email

author_email is not always in the WXR file
Alias `wp theme uninstall` to `wp theme delete`

Even though themes don't have an uninstall process, plugins do. This
brings them to closer interface parity.
Merge pull request #1571 from wp-cli/uninstall-theme

Alias `wp theme uninstall` to `wp theme delete`
Report HTTP error code on failure to install

This gives the end user more information as to why their command failed.
Merge pull request #1568 from rodrigoprimo/fix-export-posts-by-category

Fix PHP notice when calling `wp export` with --category parameter
Update php-cli-tools to v0.10.3
Merge pull request #1572 from wp-cli/1493-report-http-code

Report HTTP error code on failure to install
Merge branch 'master' into 1254-same-option-pippin
Failing test condition for #1541
Also test user meta
Check that post ID exists before changing metadata
Merge pull request #1573 from wp-cli/update-php-cli-tools-0103

Update php-cli-tools to v0.10.3
Drop test for term with non-existent parent

Core's term API has changed, and no longer will permit the
pre-condition. Our code still works; we just don't need to test for it
anymore.
Explicitly install WordPress importer

This will let us catch plugin installation failures, instead of letting
the precondition pass and test fail later