fix: swift statistics return format
feat: delete block endpoint

Blocks can now be deleted, if they have no references. A status
code of 204 is then returned.
A Constraint Error is raised by the metadata driver if there
are references, and 412 is returned as status code.

Endpoint:
    DELETE /v1.0/vaults/{vault}/blocks/{blockid}
rm: dict lookup

Explicitly typecast required swift headers to int
FileSize required for Finalize File. ContentLength added to assertHeaders
Merge branch 'master' into finalize_files
Delete missing vault, block, file
X-File-Length sent in header
pep8
fix: catch KeyError Exceptions
Merge branch 'master' into finalize_files
fix: delete from storage as well as metadata

Delete from storage, and if it succeeds delete from
metadata.
Change disk storage driver to return False if the block
didnt exist and return 404 if it ain't present.
fix: Constraint Error

per Xuan's comments
Add "Constraint Error" keyword in exception msg
Merge remote-tracking branch 'upstream/master' into block_deletion_endpoint
Merge pull request #120 from TheSriram/fix_swift_headers_return

fix: swift statistics return format
Merge pull request #131 from jc7998/finalize_files

Tests: Modify finalize file with new X-File-Length header
add: x-file-length header on finalization of file
Merge pull request #130 from jc7998/delete

Delete missing vault, block, file
rm: unused imports
add: exception handling on metadata - block delete
move exception handling to controller