iotile-build should not fail if iotile-sensorgraph is not installed

There is a global import for an `iotile-sensorgraph` provided feature in [`autobuild.py:29`](https://github.com/iotile/coretools/blob/master/iotilebuild/iotile/build/config/site_scons/autobuild.py#L29)

This function is only used in `autobuild_trub_script` and `install_requires` for `iotile-build` does not list `iotile-sensorgraph`.

The import should be moved within the function that uses it and wrapped in a `try/except` so that a nice error message can be given saying that this feature requires `iotile-sensorgraph`.