Improve the modularity of blockchain

[taoeffect]

See this comment:

# result.value = JSON.parse result.value
# TODO: [BDNS] this! also, note that we're converting JSON multiple times
#       this is ineffecient and ugly.
#       organize all this code in a generic way to support all blockchains.

Let's do this in the icann branch I just created (on top of which #67 will be rebased and a new PR submitted).

@wemeetagain let's work together to do the following:

1. Create a template blockchain.coffee file that contains a class for the other blockchains to subclass (in nmc.coffee, bdns.coffee, etc.)
2. Move the subclasses into a blockchains folder that's inside of src or src/lib (not sure yet, which do you think is better?)
3. When DNSChain boots up, automagically load up all of the files from the blockchains folder. The subclasses themselves must check to see if the system supports them. If they are able to "boot up" then they automatically set themselves up to start processing queries. This can be automated a bit by having the super class have a method that must be overridden (called config:). Return null if no file can be found, otherwise return an object? with the configuration.
4. Each subclass should check to see if the blockchain configuration file has been manually specified in dnschain.conf and use it instead if so use it instead (Allow dnschain.conf to specify path to the namecoin configuration file #60).
5. Address the inefficiency and ugly issues that the TODO: comment above describes.

Ultimately, adding a new blockchain will consist of simply copying the blockchain.coffee template into blockchains/ and overriding it, and that's it. No other files should need touching.

[taoeffect]

Note: the PR for closing this issue is currently #92.

[taoeffect]

Closed, great job on this one @wemeetagain!