Add Blob builder that doesn't sponge IO

## Use case

Writing blobs with > 1 mb is difficult to do in memory
## Syntax

``` ruby
# Use StringIO
MultiGit::Blob::Builder.new
# Use the underlying IO mechanism
MultiGit::Blob::Builder.new(repository)
```
## Strategy
- Git: fork a process and write to stdin
- Rugged: ????
- JGit: use the supplied blob writer class
