to updates or changes in depedencies, folder structure and resolver
configuration. Instead of hashing name, type and version of a
dependency, hash the json representation of the dependencies returned by
the dependency resolver, which include the path to the environment and
the depedency type. This is only applied to resolvers whose cacheable
attribute is set to True (conda-only, currently).

and only activate cached environments if they exist.

and override __eq__ for ToolRequirement, to simplify
checking if ToolRequirements are already installed/cached.

and to build a tool dependency cache (if activated in galaxy.ini).

An example to install dependencies for the twobit converter:
```
import bioblend.galaxy
url = 'http://localhost:8080/'
api_key = 'admin_api_key'
tool_id = 'CONVERTER_fasta_to_2bit'
endpoint = "api/tools/%s/install_dependencies" % tool_id
gi = bioblend.galaxy.GalaxyInstance(url, api_key)
gi.make_post_request("/".join((url, endpoint)), payload={'id': tool_id})
```
If `use_cached_dependency_manager` is activated in the galaxy.ini,
a cached environment can be built like this:
```
endpoint = "api/tools/%s/build_dependency_cache" % tool_id
gi.make_post_request("/".join((url, endpoint)), payload={'id': tool_id})
```

This will remove a pre-existing cached dependency directory.

…lation