Added the ability to pass nb in as a buffer

I have a use case where I need to pass in a buffer object instead of a string that opens a file (so I can read a notebook from a Django file object). I made a simple backwards compatible change to the constructor that allows for passing a nb buffer directly instead of a file path.
tests for SVG mime type
overdue gitignore
Merge branch 'master' of https://github.com/adamhaney/runipy into HEAD
refactored file access out of NotebookRunner
updated gitignore
version bump
+credit