#### Expected behavior
`blockchain.stream()` and `blockchain.ops()` should stream both regular and virtual Steem operations. Via optional arguments, it should be possible to stream only virtual operations.

#### Actual behavior
`blockchain.stream()` and `blockchain.ops()` only stream regular operations. Virtual ops like `author_reward`, `curation_reward` and others are not contained in the stream.

`blockchain.ops()` lists an optional parameter `only_virtual_ops`. If set, `blockchain.ops()` fails with

```
TypeError: blocks() got an unexpected keyword argument 'only_virtual_ops'
```

#### How to reproduce

```python
from beem import Steem
from beem.blockchain import Blockchain
s = Steem(node='https://rpc.buildteam.io')
b = Blockchain(s)
for op in b.stream():
    if op['type'] == "author_reward":
        print(op)
        break
```

This script never completes, because none of the ops yielded by `stream()` is a (virtual) "author_reward" op.

The same code with `steem-python` completes within seconds:

```python
from steem import Steem
from steem.blockchain import Blockchain
s = Steem(nodes=['https://rpc.buildteam.io'])
b = Blockchain(s)
for op in b.stream():
    if op['type'] == "author_reward":  
        print(op)
        break
```

##### Stacktrace for `virtual_ops_only`
```python
>>> from beem.blockchain import Blockchain
>>> b = Blockchain()
>>> for op in b.ops(virtual_ops_only=True):
...     print(op)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.6/site-packages/beem/blockchain.py", line 323, in ops
    for block in self.blocks(start=start, stop=stop, **kwargs):
TypeError: blocks() got an unexpected keyword argument 'virtual_ops_only'
```

##### Environment
```
# python --version
Python 3.6.4
# beempy --version
beempy, version 0.19.26
# steempy --version
steempy 1.0.0
```