Change Twitter share to use tags as hashtags.

Changes the twitter link to use tags instead of categories when
creating the hashtags in the link.
Also removes spaces in tags with multiple words.

For example:

post tags           | twitter hashtags
-----------------------------------------
[open source]       | #opensource
[java]              | #java
[open source, java] | #opensource #java
----------------------------------------
Merge pull request #79 from dacamo76/post-tags-as-twitter-hashtags

Change Twitter share to use tags instead of categories as hashtags.
Replace sitemap.xml with official Jekyll Sitemap gem
Update
Remove reference to sitemap.xml template
- Using Jekyll Sitemap gem instead
Use github-pages automatic sitemap generation

Merge upstream so-simple-theme updates.