add ability to create/update attachments using HTTP PUT

Allows for adding/updating attachments using simple curl command:
curl -XPUT .../db/doc_id/attachment_name?rev=document_rev_id --data-binary @attachment_file
changes based on PR comments

fix issue where 409 not generated when rev supplied for new doc
added test cases to verify 409 returned when expected
add support for If-Match header in addition to rev request param
Merge pull request #233 from mschoch/support-manual-attachment-api

add ability to create/update attachments using HTTP PUT