Added support for web manifests (closes #29)

Proposed is based on style-src's text. Both Chrome and Gecko need this. 

See: https://code.google.com/p/chromium/issues/detail?id=409996
See: https://bugzilla.mozilla.org/show_bug.cgi?id=1089255
Reduced text bit based on feedback

Updated text after discussion with @hillbrad here at TPAC.
Merge pull request #66 from marcoscaceres/patch-1

Added support for web manifests (closes #29)
CSP: Big texting the spec.
CSP3: Clean up the 'manifest-src' directive.