Debug notices

We are currently in the process of integrating MarketPress with a much larger project. As part of our development process, we work with WordPress' WP_DEBUG features enabled on our development and staging servers.

After installing and activating MarketPress, we're seeing a number of Notices, mostly from attempts to access non-existent array keys. According to this forum post that mentions the same issue <https://premium.wpmudev.org/forums/topic/marketpress-breaks-with-wp_debug-enabled>, it was on the to-do list as of 7 months ago but still seems to be an issue as of v2.6.1.

As part of our development efforts we are working to patch these warnings on our own and would like to know if you would be interested in incorporating our patches. If so, please let us know the best way submit the patch/diff files and continue this discussion.