Py-Jupyter Cadabra2 Kernel

Features:
  - latex filtered output
  - syntax highlighting and indentation with CodeMirror
  - code completion on tab press
  - sandboxed and sequential execution

Todo:
  - images/figures
  - code inspection with ? (at the moment can only use `help`)
  - input html widgets
  - neaten lexers; momentarily written as learning so not very clean
  - magic commands for kernel control 
