Add an ipython magic for snakeviz.

To use the snakeviz line/cell magics run %load_ext snakeviz and then
%snakeviz or %%snakeviz. Works on actual code (not filenames) in the same
way one would use %prun and %%prun.
Renamed magic.py to ipymagic.py.
Merge pull request #20 from jiffyclub/ipython-magic

Add an ipython magic for snakeviz.