document bash completion support
expand sieve command line documentation

- add expanded paragraph for "dbsake help sieve"
- add help summary string to each sieve option
cleanup "dbsake help uncache"

- clarify command summary
- added expanded paragraph about the purpose of uncache
rename decode / encode -> decode-tablename / encode-tablename

- decode/encode were too generic and other sorts of decoding/encoding
  might feasibly be added later which might otherwise cause
  confusion.
use curl -s rather than curl in README.rst

- this is cleaner output and less confusing for the uninitiated
don't use ctx.fail for sandbox errors

- this repeats usage and treats any sandbox failure as a usage
  failure which is confusing.  Now the sandbox error is emitted
  on stderr and dbsake fails with a non-zero error.