Added the ability to directly compile and run ethereum assembly using
the evm utility: `evm run <file>`. This is equivalant to `evm compile
<file> | evm run`.
* Improved the standard evm tracer output and renamed it to WriteTrace
which now takes an io.Writer to write the logs to.
* Added WriteLogs which writes logs to the given writer in a readable
format.
* evm utility now also prints logs generated during the execution.
