added __exit__ and __enter__ to EJDB obj to use inside with constructs.

for the ones that are used to query mongodb inside **with** constructs
and miss to automatically close the DB. 
(and to mimic more closely Mongo usage)

example:
```python
import pyejdb
def create(entries, collection='mycoll', dbname='mydb'):
    '''
    creates a new instance of ejdb with the content of a json file or with the list of dictionaries passed in entries.
    '''
    import  json
    with pyejdb.EJDB(dbname, pyejdb.DEFAULT_OPEN_MODE | pyejdb.JBOTRUNC) as db:
        if isinstance(entries,str):
            with open(entries,'r') as f:
                    entries = json.load(f)

        for addme in entries:
            db.save(collection,addme)

if __name__=="__main__":
    import sys
    create(sys.argv[1])
```