Add support for using Jetty ProxyServlet

This commit adds a function called `add-proxy-route`, which can
be used to configure certain paths to be proxied to other hosts.
Refactor proxy tests

This commit introduces a macro to simplify the proxy tests; this
will allow us to write several more tests w/o duplicating all
of the proxy setup code.
Add support for HTTPS when proxying
Add support for specifying scheme and ssl config for proxy target

This commit adds some additional fields that you can specify
when configuring a proxy target; specifically, you may now
include a `:scheme` to force http or https (rather than using
the scheme of the original request).  You can also specify
an `:ssl-config` map, to specify the pem files to use for
the ssl context.
Add docs for new `add-proxy-route` function
Add a few more tests, cleanup based on PR feedback
More explicit checks on host/port/target params
Add Jira ticket number for prismatic cleanup
Extract common ssl config from tests
Split out optional proxy config fields into separate argument
Update docs to reflect add-proxy-route API changes
Update README.md
Merge pull request #14 from cprice404/feature/master/PE-3365-add-proxy-support

add proxy support