Add user agent in request for HtmlProcessor

[davidfrigola]

Add useragent as HEADER
Configurable (see http://docs.python-requests.org/en/latest/api/ for request and optional parameters in the get method)

Add a constants list for the available agents
Default user agent (mozilla or whatever)
Random pick?

[davidfrigola]

A better approach is use a HeadersProvider injected in config at processor startup.
If none provided, add a default one
If provided, use it
Some util providers:

• DefaultAgentHeaderProvider
• RandomAgentHeaderProvider - With a list of headers
• FixedHeaderProvider - A fixed header (configured in the provider config)

Good to have:
A MultipleHeaderProvider that gets a list of headerproviders and joins all the results from each provider into a headers list for the request (may be for next Milestone)

[davidfrigola]

Added a __getHeaders methon in the HtmlProcessor, just for useragent.
Related with #25 for next milestone.