data resolver and fetcher

[marianoguerra]

for libs, templates, dependencies, projects a way to search fetch and store those components is needed.

I think a generic API with implementations for stuff like github and bitbucket (to start with) should be useful.

the idea is to have a list of places to search and then look on those paths for project.pbt files or for recursive path lists (for example one repo is a list of folders where heach is a template).

then the ability to fetch and store if needed, for example, for a plugin it will fetch it and put it on the plugin folder of pbt, for templates in the template folder and so on.

this functionality should be reusable and extensible, for example

pbt search plugin foo
pbt search template foo
pbt search project foo

where the name after search can be registered in the search command, and then

pbt install plugin foo
pbt install template foo

to fetch it and store it locally on the place it should go.

in the long term we should play nice with exprire headers, etags, project.pbt versions and similar things to avoid fetching everything every time.