script bombs if comments include unicode

@cpennington 

```
./scripts/release.py --previous 25958e87a2ee687ecd7d76e6a94739172714bc45 --current a78400a8d887aedc025aeb7d0490bcdf29be48e4 --date 2013-12-18
```

```
Traceback (most recent call last):
  File "./scripts/release.py", line 167, in <module>
    main()
  File "./scripts/release.py", line 164, in main
    print(generate_table(commit_range, include_merge=args.merge))
  File "./scripts/release.py", line 111, in generate_table
    verified="",
UnicodeEncodeError: 'ascii' codec can't encode character u'\u2019' in position 25: ordinal not in range(128)


.encode('ascii',errors='ignore')
```