Change the language of the client

Added an `attr_accessor` with the language. This setting is used in the connection to theTVDB, which allows people to set a custom language, in my case: nl.

The default language remains english (en). 

## Usage
Example: Dutch

    tvdb = Tvdbr::Client.new('API KEY')
    tvdb.language = "nl"
    dexter = tvdb.fetch_series_from_data(:title => "Dexter")
    dexter.overview # => "Gebaseerd op de 'Darkly Dreaming Dexter' en 'Dearly Devoted Dexter' boeken van Jeff Lindsay, gaat deze thriller over Dexter Morgan. [...]"

Example: German

    tvdb = Tvdbr::Client.new('API KEY')
    tvdb.language = "de"
    dexter = tvdb.fetch_series_from_data(:title => "Dexter")
    dexter.overview # => "Dexter Morgan, Blutanalyst bei der Polizei von Miami, geht nachts einer etwas anderen Tätigkeit nach. [...]"

Example: English (default)

    tvdb = Tvdbr::Client.new('API KEY')
    dexter = tvdb.fetch_series_from_data(:title => "Dexter")
    dexter.overview # => "Based on Jeff Lindsay's novels \"Darkly Dreaming Dexter\" and \"Dearly Devoted Dexter\", this crime thriller follows Dexter Morgan, a forensic blood spatter expert for the Miami Metro Police Department. [...]"