Downloadable list of live issues

Following a request from chx this morning on the dev list, here is a function which, given a set of projects, priorities, categories, and states, will return a downloadable (JSON) result containing the text of the issues, and the text of their comments and attached files.

A few notes:

• it is a first draft, so it is not even in patch format and the function name needs to be changed depending on which module this would actually go in
• the coding style is mine, not Drupal, so in case this is considered to become integrated as a feature, someone will have to convert it to Drupal style
• it might be a good idea (or not ?) to compress the results
• for larger sites, the query is heavy and so is the action of fetching the results, so it might be necessary to make use of a multi-pass mechanism with data accumulation and a deferred download