server-side nested rendering

[matteofigus]

Fixes #224

The plan is to expose in the context object 2 new methods, similar to the node.js client:

context.renderComponent(componentName [, options], callback)

with options containing optional params parameters, headers, version and callback with a signature of (error, html)

context.renderComponents(components [, options], callback)

with components array of objects containing mandatory name and optional version and parameters
and options containing optional params headers and callback with a signature of (error, html)

[jankowiakmaria]

LGTM but I think I'd like to be added:

• more unit tests for rendering multiple components (currently it's always one in tests with nestedRenderer.renderComponents) with and without component specific and shared parameters,
• and acceptance test for renderComponents (currently there is one for renderComponent)

[matteofigus]

Ok @jankowiakmaria I'll work on that - thanks!

[jankowiakmaria]

LGTM, merging