Minor coding standard corrections
Matching const capitalization for consistency
Reverted support for per-transaction currency codes

a77520be88a6784f1ece281b3e360a562c084ac8 added support for tracking the currency code for each transaction, to cover the rare case that it might change over time. It was later decided that this is overkill, and we can simply assume that the currency code will remain the same for the duration of ticket sales.

See https://github.com/Automattic/camptix/commit/a77520be88a6784f1ece281b3e360a562c084ac8 for the full discussion.
Refactored payment_refund() arguments to abstract transaction_id

Previously transction_id had to be passed to payment_refund(), but this was bad becase differnt payment modules will require different data to be passed to them. Because of that, it's better to only pass the payment token, and the let individual modules look up whatever data they need based on it.

get_data_from_payment_token() was introduced to simplify the process for payment modules.

See discussion at https://github.com/Automattic/camptix/commit/a77520be88a6784f1ece281b3e360a562c084ac8
Added mechanism for payment modules to report which features they support

Payment modules can declare which features (such as refunds) that they currently support, and CampTix can respond appropriately (such as hiding the link to obtain a refund).
Updated attendee query to retrieve any post status

get_attendees_from_payment_token() can be used in a variety of situations, and in some cases the caller may want attendees that have been refunded, are pending, etc. The function shouldn't assume that only published attendees will be requested.
Switching feature slugs to use dashes instead of underscores

Using dashes is more consistent with Core feature slugs (e.g., post-thumbnails, custom-header, etc)
Explicitly stating post_status values in get_attendees_from_payment_token()

The previous 'any' setting would miss statuses assigned to post types where exclude_from_search is true.
Renamed get_data_from_payment_token() to get_post_meta_from_payement_token()

The new name is more explicit, so that you know the data is pulled from post meta, without having to read the source.
Adds numberposts param to get_attendees_from_payment_token()

Previous numberposts wasn't set in the get_posts() query, so it would default to the 'posts per page' option in Core, which is unpredictable. The new default is to query for all attendees, but the calling function can specify a different number if needed.
Cached the results of the database query in get_attendees_from_payment_token()
Replaced numberposts with posts_per_page in get_posts call
Setting posts_per_page to always be -1 for simplicity

Since we're caching the result, it's simpler to just always use -1 rather than let the caller pass in an arbitrary number. The performance penalty from fetching a few extra posts is insignificant, and it avoids a situation where get_posts() would be called multiple times with different posts_per_page values.
Merge branch 'refunds', which has several updates and refinements related to refunds