added extract method

[willmcgugan]

Adds an extract method to base, to help with PyFilesystem/s3fs#18

It should allow 'downloading' of files without the need for a temp file. The default is a simple file copy loop.

Open to suggestions re naming. Is extract appropriate?

@althonos

[coveralls]

Coverage increased (+4.0e-05%) to 99.951% when pulling 6dbda8b on extract into 84c0f7d on master.

[coveralls]

Coverage increased (+4.0e-05%) to 99.951% when pulling 801c78d on extract into 84c0f7d on master.

[coveralls]

Coverage increased (+4.0e-05%) to 99.951% when pulling 801c78d on extract into 84c0f7d on master.

[althonos]

The code looks good (just had a quick glance), and the use case seems generic enough to be reused in other filesystems.

About the name, it could be confusing: tarfile method extract is actually closer to an open method (return a read-only file handle). The closest method of the standard library is probably copyfileobj of the shutil module. So maybe copyfileto or dumpto could be candidates as well (using the to suffix to emphasize the FS is writing to a file handle).

[willmcgugan]

Good point re the name. I've noticed a pattern that probably dictates the name. We have setbytes, settext, setfile, and getbytes, gettext. But no getfile.

I think this extract method is exactly what I would expect getfile to be. i.e. the logical inverse of setfile.

[coveralls]

Coverage increased (+4.0e-05%) to 99.951% when pulling 9ea6066 on extract into 84c0f7d on master.

[coveralls]

Coverage increased (+0.001%) to 99.952% when pulling dd3a536 on extract into 84c0f7d on master.

[coveralls]

Coverage increased (+5.0e-05%) to 99.951% when pulling dd3a536 on extract into 84c0f7d on master.

[coveralls]

Coverage increased (+5.0e-05%) to 99.951% when pulling dd3a536 on extract into 84c0f7d on master.