String import/export

[leonardoobaptistaa]

This PR add export_model_to_binary_string and
import_model_from_binary_string to Model class and update README with new
documentation.

The goal here is to have an API to import/export models without the need to use
files.

[ankane]

Hey @leonardoobaptistaa, thanks for the PR (and sorry for the delay)! I like the idea. I think we can probably use Ruby's StringIO class to accomplish this with the existing methods.

Save

io = StringIO.new # or any other IO object
model.save_model(io)
str = io.read

Load

io = StringIO.new(str) # or any other IO object
model.load_model(io)

What do you think?

[ankane]

Cleaning up stale PRs