Headers - now with less HTML and more BBCode

FIXES issue #3 

Now the headers won't be translated into HTML code.
text in lines starting with # or ## will be surrounded by [big] BBCode tags
text in lines starting with ### (only 3 hashes) will be surrounded by [small] BBcode tags
4, 5 and 6-hashes will be ignored and the content will be treated as normal text
text underlined by both === and --- characters will be enclosed into [big] BBcode tags
Below an example table

MARKDOWN | RESULT
:----------------|:-------------:
\# test | [big]test[/big]
\## test | [big]test[/big]
\### test | [small]test[/small]
\#### test | test
test<br />==== | [big]test[/big]
test<br />---- | [big]test[/big]