Jadu API interface

Add an interface to Jadu's API. /cc @dan-palmer 

Initialise the API:

```ruby
api = Jadu::API.new(
  'https://etapi.nearlive.moj.fcos.jadu.net/api/1/'
  verify_mode: OpenSSL::SSL::VERIFY_NONE, # skip verification in near-live
  ssl_version: :TLSv1
)
```

Make a new claim request:

```ruby
response = api.new_claim(xml, 'example.pdf' => File.read('example.pdf'))

response.ok? # => true
response.to_hash # =>
  # {
  #   'feeGroupReference' => '991000185700',
  #   'status' => 'ok'
  # }
response['status'] # => 'ok'
response['feeGroupReference'] # => '991000185700'
```

Make an ET office request:

```ruby
response = api.fgr_et_office('B5 4UU')

response.ok? # => true
response.to_hash # =>
  # {
  #   'status' => 'ok',
  #   'fgr' => '131000203700',
  #   'ETOfficeCode' => 13,
  #   'ETOfficeName' => 'Birmingham',
  #   'ETOfficeAddress' => '...',
  #   'ETOfficeTelephone' => '0121 000 0000'
  # }
response['fgr'] # => '131000203700'
```

Also includes a utility class for sanitising filenames according to the API requirements:

```ruby
Jadu::API::FilenameSanitizer.sanitize('Wow! A filename!.pdf') # => 'Wow_A_filename.pdf'
```