Fake blockchain generator

Should help a bit with performance tests. Surely far from perfect but we can improve it over time.

I included simple storage performance tests for postgres that can be run like this:

    TEST_DB_POSTGRES="blah"  ruby spec/bitcoin/performance/storage_spec.rb

It generates fake chain only once, alhough it takes some time. 

<code>:no_difficulty</code> option also added, thinking more about it I like @mhanne's idea to rather have list of enabled validation rules in the network hash, but this can be done later