util: provide a string->pattern function

This function will be used when we want to include plain strings into
regular patterns.
cors: add an integration test for CORS
cors: escape origin glob pattern when building regular pattern

Previously, with `http://wwwaexample.com` as Origin, we would have
matched `http://www.example.com`. We fix that with `string->pattern`
function.
meta: escape prefix and delimiters when building regular pattern
request: escape service URI when building regular expression
Merge pull request #40 from exoscale/fix/escape-regular-expressions

Escape regular expressions