bumps version to 0.5.2
fix(bless): refactors blessed css files for IE8 and IE9

* include blessed css files via separate link tags instead of one CSS file with imports
  This is important because js polyfills like respond.js does not work with CSS @import statements.
* do not save blessed CSS files to folder
  If blessed CSS files are saved to another folder than the original CSS file, relative paths within the CSS file won't work

BREAKING CHANGE: Blessed CSS files has to be imported separately from regular CSS files in index.html like this:

    <!--[if lte IE 9]>
    <% blessedStyles.forEach( function ( file ) { %><link rel="stylesheet" type="text/css" href="<%= file %>"/><% }); %>
    <![endif]-->
    <!--[if gt IE 9]> -->
    <% styles.forEach( function ( file ) { %>
    <link rel="stylesheet" type="text/css" href="<%= file %>"/>
    <% }); %>
    <!-- <![endif]-->
doc(readme): improve migration description for template extension
doc(changelog): update changelog for 0.5.2
Merge branch 'hotfix/0.5.2'