Send User-Agent

[sarahsnow1]

Overview

This updates MapzenMap, MapzenSearch, and MapzenRouter to use a new base implementation of GenericHttpHandler to send a custom User-Agent for all requests. The User-Agent is in the format <sdk_name>;<sdk_version>;<os_version>. So for example, android-sdk;1.4.1-SNAPSHOT;7.1.2. It also begins work to simplify & standardize how developers customize query parameters and headers sent for requests.

Proposed Changes

• Introduces GenericHttpHandler interface to standardize how implementing developers customize requests (future work will expand this interface Expand GenericHttpHandler #410, Allow setting GenericHttpHandler on Mapzen API objects #411). Currently developers need to access the underlying Pelias/Tangram/Valhalla API object and set a subclass of that object's http handler. Each object has a slightly different api which is less than ideal
• Adds TmpHttpHandler to allow setting headers in tile requests (will be removed when related Tangram work is completed Remove TmpHttpHandler #409)
• Possible future work could also provide the ability to use a single GenericHttpHandler for all MapzenMap, MapzenSearch, and MapzenRouter APIs

Closes #382