Upcast uptime from syscall

On arm and i386 the uptime value is int32 rather than int64,
so upcast to satify the function return value.
Merge pull request #41 from bz2/upcast_uptime

Upcast uptime from syscall
jsonhttp: new package
Merge pull request #42 from rogpeppe/001-jsonhttp

jsonhttp: new package
jsonhttp: use correct errgo package
Merge pull request #43 from rogpeppe/002-fix-errgo-import

jsonhttp: use correct errgo package
update to use gopkg.in/errgo.v1
Merge pull request #44 from rogpeppe/003-errgo.v1

update to use gopkg.in/errgo.v1
Move all the interfaces into one file.
Add missing doc comments to the interfaces.
Factor out Doc, DocStorage, and MetadataDocStorage types.
Add fake storage implementations for testing.
Move the simple implementations into their own package.
Clean up the wrapper tests.
simple -> basic.
Init docs in NewDocStorage().
Include the filename in error messages.
Add a missing doc comment.
Add more detail to a doc comment.
Drop unnecessary comment.