Adds slim template support

[keichan34]

Adds support for slim-lang.

I was considering whether to make Slim a hard dependency or not, but eventually decided against it. I think Haml should probably be made an optional dependency too, but that's out of the scope of this PR.

[coveralls]

Coverage remained the same when pulling ff29951 on keichan34:slim-support into 3e043a6 on kernow:master.

[coveralls]

Coverage remained the same when pulling d78d959 on keichan34:slim-support into 3e043a6 on kernow:master.

[kernow]

Thanks @keichan34, optional haml loading is something I've been planing to add but haven't got around to yet. Could you remove slim from the Gemfiles and add it as a development_dependency in the gemspec file, once that's done I'll merge it in.

[keichan34]

Fixed.

One thing that might not be a good idea is making haml an optional dependency, while leaving it default... But changing the default to ERB is also not a good idea because it would break users' apps.

I don't like having the haml code in my project when I'm not actually using it for anything, but it should probably be pushed back to a big-number release per semantic versioning.

[kernow]

Yes I agree with switching to erb as the default and only loading haml if available. One way around this would be to release a version of the gem that issues a warning if haml is used without it having been specified in the config block. Then at a later time switching the default to erb.

[keichan34]

Sorry about the blank lines and the one I forgot to take out of the root Gemfile, just removed them now.

[coveralls]

Coverage remained the same when pulling f465abb on keichan34:slim-support into 3e043a6 on kernow:master.

[kernow]

@keichan34 it's not released yet but the work is complete to switch to erb as the default template parser 1c57120

0.3.3 has deprecation warnings

[keichan34]

@kernow Thanks for the heads up! Keep up the great work :)