Feature: Ability to attach files to issues (attachments)

This fixes #304.

Add the ability to add attachments to issues/comments.
A new button was added below the text area of comments/issues, which allows the upload of multiple files.

Files get listed below the text of an issue/comment. Hovering over the files gives a little preview (for images).
If an issue/a comment gets deleted the files attached get deleted too.

There are ~~4~~ **5** new config values that controll the uploads. (under [attachment])

I'm not good with styling, so the CSS should probably be adjusted :-)
Here an little image:

![attachments](https://cloud.githubusercontent.com/assets/6443781/3688434/3fec4168-1336-11e4-8cbd-d1badf1a180b.PNG)

I think add tag `xorm:"TEXT"` would be better in case file path is longer than 255