Refactoring of attachment compression to allow for content-encodings other than gzip

The following patch does:

1) Changes the internal implementation of the attachment compression feature to allow the support of content encodings other than gzip. After this adding support to other encodings (deflate and compress) is trivial.

2) Removes the property "gzip_length" from an attachment stub, introduced in ticket 639, and adds 2 new ones: "encoding" and "encoded_length".

3) Renames the doc query parameter ?att_gzip_length=true, also introduced in ticket 639, to ?att_encoding_info=true

All tests, Etap and JS, are passing. Also tested this code with an existing 0.11 DB file.

This patch is for trunk only.