Updates the library, improves naming and some types

[anton-k]

Hello! Thanks for the library!

I've made some updates, hope you will find them useful:

• Builds with stack
• Scientific instead of Double for prices
• UTCTime instead of string for timestamps (i've made the proper conversion)
• Adds some useful instances for types (Eq, Ord, Data,Generic)
• Adds top-level module for easy import of the whole library
• Adds wrapper DayPeriod
• Renames some types: there were some conceptional glitches with original API:
  Ticker to Symbol, - since with getSymbols we get probably symbols.
  Symbol to SymbolDetails
TickerData to just Ticker
• Adds example of usage to README

Small example (it can be executed in the repo with stack ghci):

import Bitfinex

right = fmap (either (error "It's left") id)

main = do
  sym <- fmap head $ right $ getSymbols
  ticker <- getTicker sym
  stat <- fmap head $ right $ getStats sym

  print sym
  print ticker
  print stat