Optimized _all_docs and _bulk_get by not buffering entire response

Instead, serialize and write each response item without storing it.
Removed obsolete/unused "changes" view.
Allow "_design/..." as a valid document ID

Adds an exception to the rule against doc IDs starting with "_",
for compatibility with CouchDB.
Added a unit test.
Added dump-view handler for debugging purposes

On the admin port, /dbname/_dump/viewname returns the contents of the
given view (either all_bits, all_docs, channels or access) formatted as
an HTML table.
(Related to #52, but not identical)