implement alerts

system, email, and twitter seem best. SMS?

```
# system alert. bell rings, notifications, etc
workflow --alert 

# twitter alerts
workflow --alert @deanmalmgren

# email alerts
workflow --alert dean.malmgren@datascopeanalytics.com
```

Should it be possible to specify this from the YAML configuration, either on a per-task basis or on a workflow basis? Per-task basis might look something like this:
```yaml
---
creates: data/pdf.csv
depends: 
  - src/pdf.py
  - data/raw.dat
command: python {{depends|join(' ')}} > {{creates}}
alert: @deanmalmgren
```
Workflow basis might change the overall organization of the YAML to have a `tasks` key in addition to an `alert` key (and any other configuration options that are useful:
```yaml
alert: @deanmalmgren # received when entire workflow is complete
tasks:
  ---
  creates: data/pdf.csv
  depends: 
    - src/pdf.py
    - data/raw.dat
  command: python {{depends|join(' ')}} > {{creates}}
```