Responsive layout

I've replaced 960grid css library with the more up to date Twitter Bootstrap. This has allowed the hackspace site to be responsive across all device sizes.

For mobile devices I've taken the liberty of collapsing the front page sections and the menu on sub pages.

I've also fixed a bug on the member's edit details page where length field is mandatory. It's now a hidden field.

The styling of some fonts, forms, buttons and alerts has changed to take advantage of the default styles in twitter bootstrap, hopefully it's been an improvement. Along with these minor styling changes I've also taken the liberty of adding a background image in the header.