Small fixes for the dp path and support for a new mars backend.

New Features

• (#109) Adds mars backend support.

Enhancements

• (#110) New auto-hq and 'random-greedy-128' paths.
• (#119) Fixes several edge cases in the dp path.

Bug fixes

• (#127) Fixes an issue where Python 3.6 features are required while Python 3.5 is opt_einsum's stated minimum version.