Add mentions and definitions for bits

Live preview:
http://bitcointest1.us.to/en/vocabulary#bit
~~http://bitcointest1.us.to/en/vocabulary#bitcoin~~
http://bitcointest1.us.to/en/vocabulary#btc
http://bitcointest1.us.to/en/faq#wont-the-finite-amount-of-bitcoins-be-a-limitation
http://bitcointest1.us.to/en/developer-guide#term-denomination

"Bits" seems to be passing the test of time with a rough consensus in the community, quite strong support on social networks, positive comments from developers and increasing adoption by users, wallets and merchant tools. Recent experiments with changetips also seem to show bits are fixing design issues for the end user.

This pull req adds a mention for bits where other units are mentioned and a definition both in the vocabulary and devel-docs. To avoid confusion in devel-docs, microbit and millibit are renamed to microbitcoin and millibitcoin.

Maybe at some point in the future, using bits instead of mBTC in devel-docs's images and examples could also be worth considering?