Add new functions:

Added some new functions:

- `tracebacks_from_lines`
- `tracebacks_from_file`
- `last_traceback_from_file`

Sample usage:

```python
from tbgrep import (
    tracebacks_from_lines, tracebacks_from_file,
    last_traceback_from_file)

for tb in tracebacks_from_file(file('logfile')):
    print(tb)

for tb in tracebacks_from_file(file('logfile'), reverse=True):
    print(tb)

print(last_traceback_from_file(file('logfile')))
```