unit tests V2 step 1

features:

1) all tests are loaded dynamically from a vector test file: https://github.com/JahPowerBit/counterpartyd/blob/develop/test/fixtures/unittest_vector.py
All methods are potentially testable just by adding values in the vector.

2) vector test support 3 types of check:
- check if a function output is correct (eg. https://github.com/JahPowerBit/counterpartyd/blob/develop/test/fixtures/unittest_vector.py#L33)
- check if a function raises the expected error (eg. https://github.com/JahPowerBit/counterpartyd/blob/develop/test/fixtures/unittest_vector.py#L58)
- check if a function updates correctly the database (eg. https://github.com/JahPowerBit/counterpartyd/blob/develop/test/fixtures/unittest_vector.py#L62)

3) all tests are independent and can be executed separately with the command line. for instance:
`py.test --function=burn.parse --function=send.compose test/unit_test.py`
will run test only for burn.parse() and send.compose()

TODO: 
- complete the vector with all transactions
- export fixture and vector in json to be usable with another languages