accept final slash in declaring a DB's baseurl

Before the patch, if the URL for the database ends in a slash:

```
from cloudant import Account, Database, Document
account = Account("http://localhost:5984/")
response = account.login(USERNAME, PASSWORD)
account.all_dbs().json()
```
Then the `account.all_dbs()` request is made to the `http://localhost:5984//_all_dbs` URI (note double slash after the port number, with result:
``
{u'couchdb': u'Welcome',
 u'vendor': {u'name': u'The Apache Software Foundation'},
 u'version': u'46cb6a4'}
```

After the patch, both using a baseurl with a final slash or without makes `account.all_dbs()` request to the correct `http://localhost:5984/_all_dbs` URI, with the resulting good behaviour:

```
from cloudant import Account, Database, Document
account = Account("http://localhost:5984/")
response = account.login(USERNAME, PASSWORD)
account.all_dbs().json()
# [u'_replicator', u'_users', u'hello', u'test']
```