Fixing the documentation

The example now correctly shows the added `<br />`
tags. The missing `}}` were got added to the doc
for the `linenumbers` example.
Added tests to cover a few filters

`line breaks`, `linebreaks-br`, `linenumbers`,
`lower` and `number-format` filters are now tested.
Merge pull request #69 from adomokos/fixing-doc-adding-tests

Fixing doc, adding tests