add dex.decred.org exchange

[buck54321]

Add an exchanges.Exchange-implementing type for a Decred DEX. On by default.

Aggregated depth chart stacks deepest books first. Exchange colors are now static.

[chappjc]

Looks great. I think we can get this in today. Just need to deal with book purge and a few other little things.

[chappjc]

I stupidly made a tag that qualified as semver and we're stuck with it. Will want to make this go for whatever we have on either the master or release-0.1 branches. Seeing as I made that dumb tag I think we have no choice but to tag v0.1.1 at the same place as release-v0.1.1, so I'll probably do that.

[chappjc]

Two thoughts about the "Decred Markets" table.

1. Would be nice to retain some defined capitalization so that instead of "Dcrdex" it would be "DCRDEX"
2. Instead of 0 for DCR Vol., would be better to put "-" or "N/A"

[chappjc]

When selecting just the dcrdex market, something is wrong with the depth chart

vs.

[buck54321]

@chappjc It appears you're on an older commit. I've updated the name in the "Decred Markets" table. See what you think now.

I do see one more place I need to do that though.

[chappjc]

So I was. I see "dex.decred.org" now, which makes sense since there can be any number of DCRDEXs.

Something is still up with the depth chart though. And the zero kinda implies there's no volume, when we just don't have it reported yet.

[buck54321]

This is actually a pre-existing Dygraphs bug. For some reason, the left-most section is often unshaded if just one of its points is out of on the edge of the viewport. I see the same thing if I carefully zoom in on say Bittrex. It's just more apparent on dcrdex because the book only has a few price levels instead of the hundreds on other books. Once the book has more price bins, the chart will look normal.