Document balances and how to configure settlement parameters

[kincaidoneil]

In trying to setup an account that would prefund its peer, I couldn't find any documentation on what a positive/negative balance represents or how I should set the settle_to, nor what units I should use. The best I could find was this in line the ETH example:

The settle_threshold and settle_to parameters control when settlements are triggered. The node will send a settlement when an account's balance reaches the settle_threshold, and it will settle for balance - settle_to.

We need to document, within the HTTP API specification and the ilp-cli help text (and maybe elsewhere, too?):

• When settle_to gets triggered, and what a positive or negative balance represents
• What units the settlement parameters need to be in, with a couple examples (I was using an account with asset scale 2 and I tried to set settle_to to 4.52 but the CLI gave me a confusing error until I changed it to 452. We should clarify this or change the API to use the standard, arbitrary precision representation of the asset, since it's more intuitive).

Related to #120

[kincaidoneil]

The peering doc is more helpful, but still not great, and isn't linked from the README

[gakonst]

I've expanded our unit tests for that functionality here: 2f83e70. These should be helpful for someone trying to understand the usage and should probably be added as examples to the readmes