Jeversmann/email time

Loosely as suggested by @JakeWharton, this makes the timing information in Kochiku's emails more precise and useful. Information about parts or retries is omitted when there isn't any in order to sound natural. I also switched to haml in the html emails for consistency with the rest of the project.

The new emails are in the format:
Success:
``` 
The build was sharded into { parts } parts and took { wall_time } minutes. ({ idle_time }m idle, { build_time }m running, requiring { retry_count } retries.) 
Without sharding the build would have taken { linear_time * max_retries } minutes. ({ linear_time }m building, requiring { max_retries } retries.) 
```
Failure:
```
The build was sharded into { parts } parts and took { wall_time } minutes. ({ idle_time }m idle, { build_time }m running.) 
Without sharding the build would have taken { linear_time } minutes.
```