Non-greedy qualifier in <pre> regex

Fix to https://github.com/domchristie/to-markdown/issues/46
Test 'converting multiple code blocks' in nodejs
Merge pull request #47 from larixsource/issue-46

Issue 46
LIs with trailing whitespace
br should be replaced with two spaces and newline

Otherwise, it will be rendered without line break once converted back to html
br should be replaced with two spaces and newline

Updated tests
Merge pull request #53 from rasmusvhansen/patch-1

PR: br should be replaced with two spaces and newline
Merge pull request #51 from stevenweber/li-spacing

LIs with trailing whitespace
Added tolerance for images with no src attribute
Added test for sourceless image