Eliminated second argument ("defaultValue") from tangelo.accessor()

Closes #259
Merge branch 'no-accessor-default'
Added finer controls over what gets built.

Closes #253
Merge branch 'finer-cmake-control'
Bundling ws4py with Tangelo (since its version is <1.0)

Closes #255
Merge branch 'bundle-ws4py'
Improvements to the clientside steraming functions

Streaming callbacks now receive data and "finished" flag separately.

DELETE verb for streams now sends back the deleted key instead of the word "OK".

Closes #260
DELETE action on VTKWeb now returns the deleted key.

This is the same improvement made to the streaming REST API in commit
6c2b2b72cfc3418923cc38311e1891d6e5c1e9d5.
Merge branch 'improve-streaming'