Transfer information improvements and docs

[andrevmatos]

Move some things around:

• public RaidenChannel* types goes to channels/state submodule, for better organization
• public RaidenSentTransfer* types goes to transfers/state submodule
• Improve RaidenSentTransfer.status information, avoid setting it to SUCCEEDED or FAILED in the end, which doesn't tell much, and instead leave it in the most descriptive status possible, and one can rely on success and completed members to know the end result.
• Add documentation about transfer method and transfers$ observable!

A little related to #84

[codecov]

Codecov Report

Merging #288 into master will increase coverage by 0.44%.
The diff coverage is 78.94%.

@@            Coverage Diff             @@
##           master     #288      +/-   ##
==========================================
+ Coverage   97.34%   97.78%   +0.44%     
==========================================
  Files          39       69      +30     
  Lines         715     2481    +1766     
  Branches       63      522     +459     
==========================================
+ Hits          696     2426    +1730     
- Misses         10       31      +21     
- Partials        9       24      +15

Flag	Coverage Δ
#dapp	97.34% <ø> (ø)	⬆️
#sdk	97.96% <78.94%> (?)

Impacted Files	Coverage Δ
raiden-ts/src/raiden.ts	92.6% <ø> (ø)
raiden-ts/src/transfers/reducer.ts	98.31% <100%> (ø)
raiden-ts/src/channels/state.ts	100% <100%> (ø)
raiden-ts/src/transfers/state.ts	100% <100%> (ø)
raiden-ts/src/transfers/utils.ts	78.26% <55.55%> (ø)
raiden-ts/src/channels/actions.ts	100% <0%> (ø)
raiden-ts/src/channels/epics.ts	100% <0%> (ø)
raiden-ts/src/messages/utils.ts	100% <0%> (ø)
raiden-ts/src/utils/data.ts	100% <0%> (ø)
raiden-ts/src/transport/epics.ts	100% <0%> (ø)
... and 25 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update dfe87e1...5ac02bf. Read the comment docs.

[nephix]

Good job 👍