Add metadata methods

[martindurant]

Fixes #38

[martindurant]

(tests not created, recordings not updated)

[martindurant]

@yiga2 , a quick implementation that seems to work

[yan-hic]

Forgot you had the getsetters cooked up too. Nice !

[yan-hic]

@martindurant , here my quick-n-dirty test script - which passed fine:

import gcsfs

fs = gcsfs.GCSFileSystem()
bucket='gcsfs-metadata-test'

def upload(name, block_size=fs.default_block_size, metadata=None, patch=False):
    name = '{}/{}'.format(bucket, name)

    with fs.open(name, 'wb', block_size=block_size, metadata=None if patch else metadata) as f:
        f.write(b'0' * (f.blocksize - 100))

    if patch and metadata is not None:
        fs.setxattrs(name, myname='test')

    if metadata is not None:
       attr = next(iter(metadata))
       return fs.getxattr(name, attr)


if not fs.exists(bucket):
    fs.mkdir(bucket)

# small (< block), no meta, no setter
assert upload('small_wo_meta') is None

# small (< block), meta, no setter
assert upload('small_meta_creation', metadata={'myname': 'test'}) == 'test'

# small (< block), meta, setter
assert upload('small_meta_patch', metadata={'myname': 'test'}, patch=True) == 'test'

# large (> block), no meta, no setter
assert upload('large_wo_meta', block_size=2 * fs.default_block_size) is None

# large (> block), meta, no setter
assert upload('large_meta_creation', block_size=2 * fs.default_block_size, metadata={'myname': 'test'}) == 'test'

# large (> block), meta, setter
assert upload('large_meta_patch', block_size=2 * fs.default_block_size, metadata={'myname': 'test'}, patch=True) == 'test'

for f in fs.glob(bucket + '/*'):
    fs.rm(f)
fs.rmdir(bucket)

[martindurant]

@yiga2 , you are welcome to play with this before I merge

[yan-hic]

@martindurant I did - works fine ! Tested the put too.

Unless no need to hold off, may want to bundle with another (unrelated) change - if applicable - as per my comment here