Adding Support for ACL mime-types.

[boulderwebdev]

I noticed there is no support for uploading acl files since there is no well-defined mime-type. Because of this, if I try and POST an acl file into a container it will automatically add an unwanted file extension. This may be a one-line fix by adding the line

extensions['text/acl'] = ['acl']

to the top of ./lib/handlers/post.js.

[dmitrizagidulin]

The idea for uploading .acl files is to use PUT, which gets around the extension thing.