Fix part 2 of issue #669#issuecomment-35450820 - slashes added in front of values

Escaping no longer needed as changed from double to single quotes
Fixed issues with import of WPSEO native settings and other import/export page related issues

Fixes issue#669#issuecomment-35450820 part 3

Bugs from v1.5:
* Clean imported settings on import
* Fixed some bugs in the clean up routines
    - `wpseo` presstrend options value error
    - better cleaning of `wpseo_titles` for variable variable keys while still retaining possibly valid keys for post types/taxonomies which may not be registered yet
    - wpseo_internallinks taxonomy/post type values which could be valid were reset to default too easily
    - no need to continually change the fbconnectkey
* Fixed potential 'array index not set' issues in WPSEO_Options

Bugs from previous WPSEO:
* Fixed: Use native WP `unzip_file()` rather than calling `PclZip` directly +using the uploads directory rather than `WP_CONTENT` fixes import failure/permissions issue
* Fixed: On failure or import, uploaded file/created directory would not always be removed
* Fixed: Better error message feedback to user on import failure
* Fixed: Rest of page after 'export settings' would never load when export button had been clicked

Also:
* Readme: Added specific credits to testers which have send in actionable v1.5 bug reports
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