Bucket: add string representation and equality method

[GreatBahram]

Hello there,

I added these changes for multiple reasons,

• First of all, when I'm dealing with buckets they lack string representation which is handly some use cases:

In [4]: client.list_buckets()
Out[4]: 
[<minio.datatypes.Bucket at 0x7f15f097dc70>,
 <minio.datatypes.Bucket at 0x7f15eaae73d0>]

In [5]: # It could have been something like this:
Out[5]: [Bucket('a'), Bucket('b')]
# or this scenario:

In [6]: for b in client.list_buckets():
   ...:     print(b)
   ...: 
<minio.datatypes.Bucket object at 0x7f15eb4c4cd0>
<minio.datatypes.Bucket object at 0x7f15eaedda00>
# Instead:
In [7]: for b in client.list_buckets():
    ...:     print(b)
    ...: 
a
b

But more importantly, I wanted to check if a specific bucket exists inside a list of buckets, I know I can use bucket_exists method, but it's good to have an equality method for bucket object:

In [7]: 'a' in client.list_buckets()
Out[7]: False
In [8]: 'a' in client.list_buckets() # when you have __eq__ method for bucket obj
Out[8]: True

[GreatBahram]

I also added __hash__ method, because in the past it was possible to use bucket objects inside a set or a dictionary.

[GreatBahram]

Do you think it's a good idea to add these methods for the Object class as well? @balamurugana