Add support for PostgreSQL HSTORE data type

[KobusEllis]

Adds support for postgres hstore data type by mapping it to a BTreeMap<String, Option>. Using a BTreeMap rather than a HashMap ensures that the keys remain sorted. This is helpful for auto generated history data. Previously discussed as #151.

[abonander]

I'd at least like to start with something that gives us more room to evolve the API later (this also makes it clear that the expected SQL type is hstore):

pub struct PgHStore(pub BTreeMap<String, Option<String>>);

impl Deref for PgHStore {
    type Target = BTreeMap<String, Option<String>>;

    ...
}

impl DerefMut for PgHStore {
    ...
}

impl FromIterator<(String, String)> for PgHStore { ... }

impl FromIterator<(String, Option<String>)> for PgHStore { ... }

impl IntoIterator for PgHStore {
    type Iterator = btree_map::IntoIter<(String, Option<String>)>;

    ...
}

With this, we could add a defaulted type parameter, e.g. T = BTreeMap<String, Option<String>> to allow the use of other types without breaking existing usage (for the most part, anyway). I was thinking if we used serde::Deserialize and serde::Serialize, the user could use any map type they wanted, or even a struct.