Make swift / s3 requirements optional

[jtratner]

Turns out PBR does support extras-require, thus installation becomes:

pip install stor[s3,swift]

Would be nice for making a much smaller install for people that are only using s3.

Requires:

• move requirements to extras
• change imports to be optional
• add tests for useful exceptions when appropriate imports aren't available

Open questions:

Should stor.Path('swift://AUTH_final_analysis_prod/A/C') work when swift utilities aren't installed? (similar question about s3:// etc)