Generate precompiled, page and email layouts independently.

Only precompiled and email layouts needs to be generated into html to use internally. Page layouts only are needed to generate specific pages (sessions, sponsors, presenters, etc)

```
gem build jekyll-attendease.gemspec
```

```
gem install jekyll-attendease-0.6.2.gem
```

To install in Attendease go to the project folder and run the above command pointing to the correct .gem file. 

This PR does the following:

- compiles the pre-compiled html layouts for attendease
- creates the layouts if they are not defined, uses the existing otherwise.
- compiles the pre-compiled email layouts for attendease
- creates the layout if it is not defined, uses the existing one otherwise.
- generates the layouts for page generation
- moves the layouts for page generation to `_attendease/layouts`. This keeps all assets for page generation in `_attendease`