Httpclient

I think `httpclient` is the one. It's thread safe afaik:

```ruby
client = Firebase::Client.new(url, secret)
threads = 3.times.map do
  Thread.new{ client.get('test').body }
end
threads.each(&:join)
# doesnt blow up
```

Seems to be 2x faster than `patron`. Here's using the same [benchmark gist](https://gist.githubusercontent.com/aj0strow/699012f8014793190585/raw/819484c077bdd2e8a4246f40b155215d7d1919c8/bench.rb). 

Before with `typheous`:

```
       user     system      total        real
curl          0.000000   0.000000   0.050000 (  0.306820)
firebase gem  0.030000   0.010000   0.040000 (  1.302462)
patron        0.010000   0.000000   0.010000 (  0.508994)
```

After with `httpclient`:

```
       user     system      total        real
curl          0.000000   0.000000   0.050000 (  0.494802)
firebase gem  0.010000   0.000000   0.010000 (  0.248934)
patron        0.030000   0.010000   0.040000 (  0.461365)
```

Should close:

https://github.com/oscardelben/firebase-ruby/pull/35 https://github.com/oscardelben/firebase-ruby/issues/32 https://github.com/oscardelben/firebase-ruby/issues/31 maybe https://github.com/oscardelben/firebase-ruby/pull/33 ?
Change looks good! Only have a couple comments here:

* Maybe we should add `@client.default_header['Accept'] = 'application/json'` for completeness' sake.
* Any thoughts on setting default `connect_timeout` or `connect_timeout` values? Or at least making them configurable? I like to do a lot of queries to one persistent endpoint, so it's useful to me to specify a long timeout. Maybe we can pass an options hash to `HTTPClient`?