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.