improve perfs
huge refactor: simpler ops, remove aleph/lamina, streaming ops

This work introduces a few new concepts:

- introduce "descriptors" for common entities, allowing fast
  access to commonly used parts of the system
- requests now get a target descriptor assoc'd during preparation
- permission validation has moved to its own namespace
- input and output streaming have moved to their own namespace
- streaming operations now support InputStream and OutputStream
- no more dependency on aleph or lamina
- web service now provided by jetty9 through jet.

Additionaly, the code should now be more straightforward to browse
through, especially the upload and download bits.

A huge thanks to @mpenet for his work on alia and jet.
Merge branch 'master' into feature/stream-ops

Conflicts:
	src/io/pithos/operations.clj
Merge branch 'master' into feature/stream-ops

Conflicts:
	src/io/pithos/operations.clj
Merge pull request #26 from exoscale/feature/stream-ops

huge refactor: simpler ops, remove aleph/lamina, streaming ops