Adding section about the client-server structure.
Added images for intro.
Updated the getting started w/ a note that models use HTTP-based services and don't connect to databases. Also, finished section about what is different in Mojito.
Fixing reST formatting.
Fixed image.
Merge branch 'develop' into docs_getting_started_update
Added a line about using the REST class.
Merge pull request #1271 from zhouyaoji/docs_getting_started_update

[docs] Added a section to the Mojito overview about the differences between Mojito and other Web app architectures.