Use global variable for _julia_runtime

Previously we monkey-patched sys.  A global in the julia module seemed
cleaner.
use jl_init_path

This was a kwarg before but wasn't actually used
style differences
Add informative error for missing PyCall library

Yields the following error

Julia does not have package PyCall.
Install PyCall by running the following line:
    julia -e 'Pkg.add("PyCall"); Pkg.update()'
Merge pull request #24 from mrocklin/dev

Style changes