replace theme model with plain ruby object

register themes via configuration instead of creating them in database
migrations.

BREAKING CHANGE: the default theme has to be registered in the
pageflow initializer.
automatically register theme css files for precompile
theme generator

add a rails generator to copy the default theme into the main app for
modification
editing theming attributes through account admin

allow changing imprint/copyright link text/url of account's nested
default theming
ensure theming belongs to account

* associate theming with account on creation
* allow entry theming selection based on account name
fix column name in drop themes migration
Merge pull request #45 from tf/theme-poro

Turn Theme into Plain Old Ruby Object