JSON-RPC

Changes made in this pull-request:

- jsonrpc api with all methods available from javascript-api
- added cors headers to jsonrpc
- separated libethrpc
- Javascript jsonrpc client, with the same interface as Javascript bindings in EthereumBrowser, so there is now possibility to run/debug code using eth object in Chrome/Firefox and other browsers
- here is a playground - https://github.com/debris/eth-client-example
- added async setters/getters for javascript properties
- tests for most of jsonrpc methods

* api is not yet unified with Go client api
This is the same as TransactionSkeleton. Can TransactionSkeleton be abstracted to a common eth type and be used instead of two transaction skeleton types?