Use "tags" as base field rather than "tags_array"

@lgs I did some refactoring so that the underlying Mongoid field is now called "tags" (type Array) rather than "tags_array". I think this is more natural/clean as "tags" now accesses MongoDB directly without any magic/type conversion in the gem, however, it does have potential to break existing code hence I bumped the version to `1.0.0` according to semver.

To give an example:

```ruby
# OLD

my_model.tags         #=> returns String "foo bar baz"
my_model.tags_array   #=> returns Array ["foo","bar","baz"]

# in MongoDB, the "my_model" document has a field called "tags_array"


# NEW

my_model.tags          #=> returns Array ["foo","bar","baz"]
my_model.tags_string   #=> returns String "foo bar baz"
my_model.tags = ["foo","bar","baz"]   #=> persists tags Array ["foo","bar","baz"]
my_model.tags = "foo bar baz"         #=> persists tags Array ["foo","bar","baz"]

# in MongoDB, the "my_model" document has a field called "tags"
```

All tests passing and code has been synced to your latest commit.

It's your call whether to merge it or not, either way I'm fine :beers: