Implemented BlockHelper

- BlockHelper: the base for view-level helpers with custom DSL, e.g. form builders
Fixed output buffer capture, implemented basic form tag

- fixed major template capturing bug, now properly working for ERb and Haml templates, with no dependance on Sinatra::OutputBuffer
- implemented basic form tag helper
Clean up: 'sinatra-outputbuffer', tags formatting

- removed 'sinatra-outputbuffer' dependency
- better tags formatting (not for Haml though)
Moved test routes and views from aerogel into development app.