Fix basic serialization problems

[theoreticalbts]

Due to numerous bugs, the serializer doesn't yet work with the following simple transaction:

{
   "ref_block_num":0x1234,
   "ref_block_prefix":0x56789abc,
   "expiration":"2018-01-03T12:00:00",
   "operations":[
      ["transfer",
         {"from":"alice",
          "to":"bob",
          "amount":"7.625 STEEM",
          "memo":"transfer some tokens"
         }]],
   "extensions":[],
   "signatures":[]
}

I will fix the specific bugs triggered by this test case:

• Allow dates to be specified as str
• ArgumentError exception class used throughout Serializer is not defined
• static_variant does not serialize properly
• The exception handler which prints debug info in fields() uses too specific exception type
• asset isn't just another class, its JSON serialization is a special, special snowflake
• Add handler for signed_transaction (that's where an extra zero byte in the get_transaction_hex() serialization comes from)
• Add example script to compare against signed_transaction