Issue #597 Handlebar helpers

Handlebar-helpers package, implements:
```html
{{getSession 'foo'}}
```
```html
{{#if sessionEquals 'foo' 'bar'}}
```
```html
{{#with findOne 'myCollection' '{}' }}
```
```html
{{#each find 'myCollection' '{}' '{ "sort": { "createdAt":1 } }'}}
``` 
*+Tests, docs and an example*