Documentation for all ops that have been defined so far. Includes numerous edits by @ECouzens.

* Refactored to run builds as user, not as root.  Added make install.  Added parallel build.  Expanded make clean to remove BUILD directory.  Moved log generation to makefile, so Jenkins job can let make do all of the work.

* Changes/fixes to address code review.  Mainly adding double-quotes around variables and using $(...) expansion where appropriate (and semantically identical to back-quotes).

* Add double-quotes for variable expansion safety.  Write error message in run_as_user.sh to stderr.

* Remove chown_files.sh, made obsolete by running as user inside docker container.  Add set -e and set -u to run_as_user.sh, for further protection.

* Elementwise sign operation

* Formatting

remove unused file call_frame

add sha1 of eigen download file so it does not have to redownload if …

change all passes to take shared_ptr rather than naked pointer

missing include

fixing vector length

format fixes

avoid repeating input values in a test

use input

trig/hyper funcs

* Formatting; fix erroneous "type_prop" classification for reduce and slice tests

* Clarify comment

* Add step parameter to slice, with type checking but not (yet) implementation in VM

* Add elementwise exp to VM

* Formatting

Bob/warnings

* Type checking for reshape op

* Formatting

* Implement reduce op for matrices and smaller in VM

* Formatting

* Missing include for Ubuntu

* Remove IndexBuiltin class

Docker: Add more deps

Add a minimal Backend API and make the interpreter use it.
read/write tensors (for frameworks)

* cleanup

* fix node includes