Card devexp_395 Added documentation for STI.

I added the cli docs and I did small flag cleaning:
* unified certain flags (namely `callbackUrl` -> `callbackURL` and `savetempdir` -> `saveTempDir`)
* added `forcePull` and `saveTempDir` to `usage` command, as these make sense there as well

@bparees @csrwng waiting for your comments.
assemble is also supposed to be responsible for extracting the tar stream coming on stdin (the tar that contains the src and artifacts), right?  (that's how we're getting rid of sti-helper)