Add `valohai.distributed` with config parsing

[ruksi]

Fixes #87

TODO:

• [x] add valohai.distributed to provide common helpers for reading Valohai distributed task configuration
• [x] add a valohai.distributed section to README

Also includes other small improvements here and there.

[ruksi]

• Removed Network structure and moved those properties one level up.
• Made ranking work deterministically with non-integer member ids and integers that don't start from zero.
• Updated README to reflect these.

[ruksi]

• yeet MASTER_MEMBER_ID and expect the master always to simply be rank 0
• rename self() to me() so it won't clash so much with the usual self
• apply the suggested refactorings

[ruksi]

never failing try/except added