Chunky Blobs

Some changes to `Rugged::Blob`

* more tests
* minor doc fixes
* fixed a bug where if negative max_lines is given to `blob.text` an empty string was returned instead of the whole text
* exclude `git_blob_id`, `git_blob_owner` from the coverage 
* added methods:

```ruby
Rugged::Blob.from_chunks
Rugged::Blob.from_disk
blob.binary?
```
* renamed `Rugged::Blob.create` to `Rugged::Blob.from_buffer`, there are multiple ways to create a blob, this is more consistent with the naming of the other methods. Having a `create` method can be seen as the preferred way to create blobs, which is not the case.  


The `from_chunks` reader implementation is lifted with minor modifications and additional type check  from:
https://github.com/sparklemotion/nokogiri/blob/master/ext/nokogiri/xml_io.c#L13