(refactor) simplified content_type retrieval

This commit changes the content_type retrieval
to use the built in method `extension_str()` instead
of the hand crafted Regex to figure out the
content_type.

It also adds a test.
(refactor): simplify file structure around mimes

This inlines the `get_content_type` function
and kills the old `response/mimes/mod.rs`
file while the old `response/mimes/mimegen.rs`
becomes the new `response/mimes.mod.rs` file.
Merge pull request #86 from cburgdorf/mimes

(refactor) simplified content_type retrieval.